Some representative Late Cretaceous Gastropods from the Pacific Coast of North America

There are about 200 reported genera of Late Cretaceous (about 100 to 65 million years ago) shallow-marine gastropods known from the Pacific coast of North America. This post focuses on a few examples of them.

Tessarolax is one of the most "eye-catching" examples. It is an extinct genus characterized by having four long digitations, used to support its shell on soft ocean-floor sediments. The above images are of three different species at oblique views. The specimens range from 70 to 120 mm (max. dimensions). Tessarolax belongs to family Aporrhaidae (pronounced "a-poor-rye-id-dee"). Tessarolax, which is only of Cretaceous age, was widespread in the world.

These two images of Tessarolax are of two different species showing how their shells relate to the ocean floor (substrate). Both specimens are approximately 50 mm, from left to right. Very careful collecting and cleaning are required in order to obtain such high-quality specimens. For the latest information on Tessarolax and also on the details about the species represented by the above-illustrated specimens, see the reference: 

Saul, L.R. and R.L. Squires. 2015. Pacific slope of North America record of the Cretaceous aporrhaid gastropod Tessarolax: Evolutionary trends, mode of life, and paleobiogeography of the genus. Contributions in Science v. 523, pp. 37–65. (pdf available for free---just "Google" the title of the paper.

Anchura is another example of an aporrhaid gastropod having an extension of its shell in order to provide stability on the ocean floor.
For more information on Anchura, see the reference:

Elder, W.P. and L.R. Saul. 1996. Taxonomy and biostratigraphy of Coniacian thorugh Maastrichtian Anchura (Gastropoda: Aporrhaidae) of the North American Pacific slope. Journal of Paleontology 70(3):381–399. (pdf available for free---just "Google" the title of the article).

These two specimens of the gastropod Xenophora look like two small stacks of pebbles. This genus, which is extant, is known as colloquially as the "carrier shell" because it collects and carries pebbles and/or other shells (by cementing them onto its own shell), in order to provide protection/camouflage for itself. In essence, it was the first fossil collector. For more information on this fossil Xenophora, see the reference:

Squires, R.L. and L.R. Saul. 2001. New Late Cretaceous gastropod from the Pacific slope of North America. Journal of Paleontology 75(1):46–65. (pdf available for free---just "Google" the title of the article).

This specimen (height 50 mm) belongs to genus Paosia, formerly known by its junior synonym name of Trajanella. Paosia has a long geologic time range of 75 million years. It is mainly known from Europe, but a lineage became established along the Pacific coast of North America for 25 million years. For more information, see the reference: 

Squires, R.L. and L.R. Saul. 2004. The pseudomelaniid gastropod Paosia from the marine Cretaceous of the Pacific slope of North America and a review of the age and paleobiogeography of the genus. Journal of Paleontology 78(3):484–500.

Trochactaeon (T.) packardi (see my Post 39 of Feb. 3, 2015): An unusual Late Cretaceous gastropod from California. The specimen on the left is 6 cm height and shows the front view. The specimen on the right is 4.6 cm height and shows the back view. This genus, which was restricted to the Cretaceous, was widespread, including California, in warm waters during the Late Cretaceous.

Alamirifica corona genus and species Saul and Squires, 2003; two views, front and back (height 37 mm). Late Cretaceous (Turonian), southern California. This specimen is exceptional because it shows all the exquisite details of its aperture. For more information about this species, see the reference: 

Saul, L.R. and R.L. Squires, 2003, New Cretaceous cerithiform gastropods from the Pacific slope of North America. Journal of Paleontology 77(1):442–453.

Pentzia hilgardi (White, 1889); two views, front and back (height about 10 mm). Late Cretaceous (Campanian to early Maastrichtian). Genus Pentzia consists of only a single species, which is found only in Washington, California, and northern Baja California (Mexico). This species has an usually long geologic time range, which is approximately 13 million years. Most gastropod species last for much shorter time intervals. For more information about this genus and species, see the reference:

Squires, R.L and L.R. Saul, 2003, New Late Cretaceous (Campanian and Maastrichtian) marine gastropods from California. Journal of Paleontology 77(1):50–63.

Igneous Rocks

Igneous rocks are derived from magma = "red-hot," molten matter emanating from inside the Earth.

The above diagram shows the basic classification of igneous rocks. A substitute rock for rhyolite is any glassy textured rock, like pumice, obsidian, or pele's hair. Another name for "dunite" is peridotite.

This cross section of the Earth' crust shows where the major types of igneous rocks form. Note: basalt = flood basalt (on the continents) and pillow basalt (on the floor of the oceans).

This diorama, which is a more graphic way of showing the table presented at the beginning of this post, shows representative pictures of the major types of igneous rocks. These specimens are mostly hand-size or smaller.

The rocks above the centimeter scale in the diorama are volcanic (extrusives) that cooled quickly from magma extruded onto the Earth's surface, whereas those below the scale are plutonic (intrusives) that cooled slowly from magma bodies inside the Earth.

For each extrusive rock, there is a counterpart intrusive rock. For example, pumice (as well as rhyolite), and their underground (intrusive) counterpart granite, generally have similar chemical compositions (i.e., they both have minerals rich in quartz and potassium feldspar). Andesite and its underground (intrusive) counterpart diorite have their own similar chemical composition (e.g., they both have minerals low in quartz and intermediately high in potassium feldspar). Basalt and its underground (intrusive) counterpart gabbro have their own similar chemical composition (e.g., they both have minerals without quartz and those high in potassium feldspar).

specimen 9 cm height

 Pumice, from the 1980 eruption of Mt. Saint Helens, Washington.

Side view of the same specimen, showing the layers of pumice.

specimen 8.5 cm height

A different type of pumice (locality unknown), showing "air holes." Pumice is so "light" in density, it can float on water.

specimen 5 cm height

Obsidian is volcanic glass with a low amount of iron, which produces the black color. Obsidian forms by rapid cooling. This kind of rock, which breaks into very sharp pieces, is used commonly for making arrowheads.

specimen 8.5 cm height

"Snowflake obsidian," a natural volcanic glass containing the mineral cristobalite (= the white "snowflake" crystals consisting of radially clustered crystals). The cristobalite formed due to the partial crystallization of the glass. This specimen is from Millard County, Utah.

specimen 7 cm long; it comes apart live a stand of hair

Pele's "hair" is a volcanic glass spun by blowing-out during quiet fountaining of fluid lava. It is named for the Hawaiian goddess of volcanoes, Pele.

specimen 22.5 cm height

This large cobble of rhyolite shows "flow banding" caused by flowage of crystals before the magma solidified.

specimen 9.5 cm height

Crystals in this piece of granite are visible because the magma cooled very slowly (thousands of years) underground. The large pink crystals are potassium feldspar. The black mineral is biotite, and the other minerals are quartz (gray) and sodium-rich plagioclase (white).

specimen 9.5 cm height

Andesite (a common rock in the Andes Mountains of South America-hence the name "andesite"). The crystals are not visible because they cooled quickly and did not grow to a larger size.

specimen 6 cm height

Diorite is the intrusive-rock equivalent of andesite. Hornblende is a common constituent, as is intermediate form of plagioclase (a mixture of both sodium-bearing and calcium-bearing plagioclases).

specimen 11 cm height

Basalt is the most common rock on the ocean floor; in fact, its makes up the ocean floor if you subtract the mud and sand that settled on top of it. The iron content of basalt makes it black in color. Basalt is also found on the Moon = the dark patches (the mare).

specimen 7m height

This is a basalt stalactite (a dripping from the top of a lava tube in New Mexico). 

specimen 13.5 cm height

This basalt-lava volcanic "bomb," while still molten, twisted in the air before hitting the ground.

specimen 4.5 cm width

Basalt can contain inclusions, called xenoliths (="stranger rocks"), which consists of a fragment of a plutonic rock. The xenolith was picked up by the basaltic magma as it moved upward toward the surface of the Earth.

specimen 3.5 cm height

Gabbro has visible crystals because it cooled slowly underground. This rock, which is very low in quartz, has abundant hornblende and calcium-rich plagioclase. Gabbro is found also on the Moon = the "white" patches.

specimen 9 cm height

This rock, called dunite, consists almost entirely of olivine.

Common rock-forming minerals

The following is a very basic discussion of minerals, which is critical for the understanding the classification of igneous rocks, the topic of the next post.

Atoms (electrons, protons, and neutrons) make up elements (92 natural ones; (examples: oxygen, silicon, potassium).

Elements make up minerals (3,500 kinds).

     Minerals consisting of pure (native) single elements are rare     (e.g., gold, diamond, copper).

     Nearly all minerals consist of combinations of elements.

Only 20 minerals are common, and they are the rock-forming minerals. Seven of these are shown below.

white ("milky") quartz, 3.3 cm in height

Quartz (made up of the elements silicon + oxygen) is the most common mineral in the continental crust of Earth. The presence or absence of quartz in a rock is fundamental in the classification of igneous rocks. For more information, see my previous two posts, which review the subject of quartz.

potassium feldspar (K-spar), 3.7 cm in height

One of the largest groups of minerals is referred to as the "feldspar group," which consists of two subgroups. One of these subgroups is the "alkali feldspars,"which contain varying amounts of the elements potassium and sodium in their composition. The mineral pictured above is one of these alkali feldspars, which are generally called "potassium feldspar" or "K-spar" until x-ray and petrographic microscope studies are made to determine the exact mineral. 

plagioclase, 3.3 cm in height

The second subgroup of the "feldspar group" is called the "plagioclase feldspars," which contain varying amounts of the elements sodium and calcium in their composition. Plagioclase is characterized by the presence of striations, which can be seen as closely spaced lineations in the upper half of the specimen shown above.

flakes of muscovite, largest flake 4 cm height

Muscovite and biotite (see below) belong to the "mica group" of minerals. Both have one-dimensional cleavage, which splits into sheets or flakes. For a discussion of what cleavage is, see one of my more recent posts. Because of its clarity, large sheets of muscovite could have served as "windows" for pioneers living in wood cabins. Muscovite is common in quartz-rich igneous rocks (granite), metamorphic rocks like (schists and gneisses), and sedimentary rocks (siltstone and claystone).

large sheet of biotite, 6.5 cm height

Biotite has a more varied composition than does muscovite. For example, biotite has iron in its structure, which causes a dark coloration. Biotite occurs in igneous rocks, as well as a wide variety of metamorphic rocks. 

hornblende, 7 cm height

Hornblende has a very complicated composition and much chemical variability, depending on the chemicals available and the temperature of formation. Hornblende has two directions of cleavage (approximately at 60 degrees and 120 degrees). This mineral is found in a wide variety of igneous and metamorphic rocks. 

olivine, 3 cm height

The "olivine" group minerals contain varying amounts of iron and magnesium in their composition. "Olivine" is most common in quartz-free rocks and commonly has a green color.

In the minerals shown above, three of them occur as solid-solution series, which form when two elements can substitute for one another in a mineral. The three solid-solution series are the
alkali feldspars, plagioclase feldspars, and the "olivine" group.

Most of the minerals in these solid-solution series are not compositionally pure end members; rather they have an in-between composition.

Minerals make up rocks (3 main types: igneous, sedimentary, and metamorphic).

Rocks make up the Earth's crust (two kinds: continental and oceanic).

In the next post, igneous rocks will be discussed and illustrated, and the chart below is presented here to show how quartz is used to help define certain kinds of igneous rocks. For example, granite (think expensive kitchen-counter tops) is rich is quartz but basalt (think lava flows in Hawaii) is low in quartz.  

Varieties of Quartz: Part 2

The previous post showed some common varieties of macrocrystalline (visible crystals) of quartz (silica = SiO2). This post shows some common varieties of quartz that do not show any visible crystals and have a dense structure, referred to as  microcrystalline  [= cryptocrystalline] quartz. Two main groups are shown here: chalcedony and opal.

Chalcedony is the generic named given to materials that are composed of microcrystalline quartz. Chalcedony includes blue chalcedony, chert, "touchstone," agate, jasper, and "tiger eye." Examples of these are shown below: 

height 5 cm, from Nsanje District, Malaw (southeast Africa) 

Blue chalcedony (commonly called blue "quartz"). Chalcedony is a microcrystalline form of quartz (silica, SiO2), composed of intergrowths of quartz and moganite, and each has a different crystalline structure. 

height 3 cm

Chert is a hard, opaque chalcedony with an amorphous microcrystalline texture. Chert has conchoidal fracture, which produces very sharp edges, especially in flint (black or brown forms of chert). That is why chert was commonly used for making arrowheads.

height 7 cm

The image above shows a sedimentary rock consisting of chert layers (in black) alternating with impure white siliceous (silica bearing) material, referred to by some geologists as "porcelaneous chert" (i.e., resembles glazed porcelain).

height 7.5 cm

"Touchstone" is a black, nodular version of chert. For at least 2,500 years, it has been used by goldsmiths to test the purity of gold and silver by the streak left on the stone when rubbed against these metals. "Touchstone" is also called the "lydite stone," also spelled as "lyddite."

sliced and polished specimens, largest one height 7.5 cm

Agate is banded chalcedony and has a wide range of colors. The bands form over time and can have different colors, depending on the impurities. Agates form commonly in areas of volcanic activity or in limestone where waters, rich in dissolved silica flow through fractures and cavities in these rocks.

height 1.7 cm

An agatized snail, whose hollow interior was filled with agate.

petrified wood (14 cm width), from Petrified Forest, Arizona

Petrified wood is, in most cases, wood replaced by chalcedony. Other minerals (like opal), however, are also known to replace wood.

height 5 cm

Jasper is an opaque aggregate of chalcedony and microgranular quartz. Jasper is an impure variety of silica, thus it can have red, yellow, brown, green, or other colors. The red color of jasper is because of iron inclusions.

polished specimen, height 3.7 cm

"Tiger eye" is a metamorphic rock consisting of chalcedony that formed by replacement of an asbestos-group mineral (called crocidolite). "Tiger eye" has a golden brown to very dark brown color and silky luster. 

The other group of microcrystalline silica is opal, which is a mineraloid consisting of hydrated amorphous silica (water content 3 to 21 percent by weight; commonly 6 to 9 percent). Opal is deposited at relatively low temperature, and it can be deposited in the fissures of any kind of rock, especially volcanic rock. Opal is the national gemstone of Australia.

Examples of "precious opal" are shown in the next two images below. "Precious opal" displays flashes of colors when exposed to light (= a form of iridescence). 

height 1 cm

height 2 cm

height 4.5 cm

This last variety of opal is "fire opal," which is bright red, yellow, 
or orange in color.

width 10 cm

Both macrocrystalline and microcrystalline quartz occur commonly in geodes, which are round rocks with a hollow cavity line with crystals. In the example above, the geode has two hollow cavities, both partly filled with druzy (macrocrystalline) quartz alternating with layers of chalcedony.

width 12 cm

Cavities completely filled with chalcedony (e.g., agate, jasper) are called nodules. In the nodule shown above, macrocrystalline amethyst mostly filled a geode. The remaining cavity was filled eventually filled completely with chalcedony. This unusual specimen was collected by my friend, the late George Edward Davis.