Proceedings of the California Academy of Sciences, Volume III, 1863-1867

by Various

BY J. D. WHITNEY.

At a meeting of the Academy in May last, I gave some account of the geology of the State of Nevada, with particular reference to the age of the stratified deposits occurring there, as determined from the collections of fossils brought from that region to the office of the Geological Survey, by J. E. Clayton, and various members of our corps. In that communication I spoke of the probable future discovery of rocks older than the Carboniferous or Devonian, in the mountain ranges near Austin. This expectation has been realized, and we are now in possession of a very interesting collection of fossils, obtained by Mr. A. Blatchley, in the vicinity of the Hot Creek Mining District about one hundred miles southeast of Austin. This collection enables us to state positively that both Upper and Lower Silurian rocks occur in that district, and that they are well filled with fossils; not less so indeed, to judge from the specimens received, than the strata of the same age in New York, Ohio, Iowa, and Wisconsin, which they resemble in a most marked degree, both lithologically and palæontologically.

The fossils from the Hot Creek District are mostly weathered out on the surfaces of thin slabs of bluish-gray argillaceous limestones, and are crowded together in the same profusion with which they have often been noticed by myself and others as occurring in the Lower Silurian shales and limestones of the Wisconsin Lead Region, around Big Bay des Noquets, and in many other localities in the country bordering on the Great Lakes.

Both the upper and lower divisions of the Silurian appear to be represented by the fossils of the Hot Creek District; but the lower Silurian seems to be much the most prolific in fossils, as is the case in Wisconsin and Iowa. The particular period to which these lower Silurian forms may be referred is the Trenton, including the Chazy, Birdseye, Black River and Trenton limestones of the New York Geologists, and the Buff and Blue limestones of the Western surveys. Nearly all the prevailing types of the Eastern rocks of this age, are represented in the Hot Creek collection, namely: Brachiopods, Gasteropods, Cephalopods, Crinoids, Trilobites, and Corals; and there are among them several of the most widely-distributed and most characteristic species of the Lower Silurian. The following have been identified: Maclurea magna, a characteristic Chazy species, and Pleurotoma lenticularis, Orthis testudinaria and Chætetes lycoperdon, all of which are abundant in the Trenton limestone of New York, and the rocks of the same age farther West. Among the fragments of Trilobites, two or three different genera may be recognized, especially Asaphus, which is represented by a species apparently new. There are also fragments of crinoids or cystids closely resembling the species figured by Hall, in the Palæontology of New York, Vol. I, as Echino-encrinites anatiformis.

The rocks containing the above mentioned fossils crop out in the sides of a deep cañon; and overlying them, at a perpendicular distance of about a thousand feet, is a series of beds containing numerous fragments of corals and crinoids, silicified and weathered out from the surface of a bluish-gray limestone, which I refer without much doubt to the age of the Niagara limestone of New York. Among the corals, Heliolites spinipora and Syringopora are recognizable; and among the crinoidal fragments are stems of what appears to be Caryocrinus ornatus.

With the exception of the Potsdam sandstone fossils, described by Meek and Hayden as occurring at the base of the fossiliferous series, in the Black Hills, no recognizable Silurian forms have been observed by geologists, in the Rocky Mountains, or anywhere to the west of them, unless possibly in New Mexico. The Silurian Series, with the possible exception of the Potsdam sandstone, seems to be entirely wanting in the Rocky Mountains proper, the Black Hills being a sort of outlier of the main ranges, and lying as far east as the one hundred and third to the one hundred and fifth meridian. Dr. Hayden says, in his paper, on the Geology and Natural History of the Upper Missouri, published in 1862, that “hitherto no indications of the existence of any other member (than the Potsdam sandstone) of the Silurian period has been discovered along the eastern slope of the Rocky Mountains within the boundary of the United States.” He considers it probable that the Potsdam sandstone is represented in the Rocky Mountains, although no fossils of that member of the series has been as yet discovered anywhere to the west of the Black Hills.

On the Mexican Boundary Survey, a few fragments of fossils were found in the superficial detritus, near El Paso, (Longitude, one hundred and six degrees) which indicated the existence of Silurian rocks in that vicinity; but none appear to have been found in place. Professor Hall remarks that “the specimens referable to strata of this age (Devonian and Silurian) are few, and they are in such condition as to give little satisfactory information regarding the rocks in place.” The specimens obtained are figured in the Mexican Boundary Report, but not described, nor is their locality accurately stated.

Dr. Newberry, in his Report in the Geology of the Colorado River region, refers the lower portion of the strata exposed in the grand cañons of that river to the Devonian and Silurian Series; but as no recognizable fossils were discovered by the Ives’ Expedition from any rocks lower than the Carboniferous, this reference can only be taken as expressing a conviction based on lithological characters and stratigraphical considerations.

In view of the above cited facts, it will be seen at once how interesting this discovery is of undoubted Silurian rocks west of the Rocky Mountains; and the more so, since we have in this remote region a recurrence of conditions and forms of animal life so closely allied to those with which we are familiar in the States east of the Mississippi. It is a very remarkable fact that these rocks have not been discovered in the Rocky Mountains; and should farther explorations fail to reveal their presence, it will throw a new light on the history of the physical development of the central and western portions of this continent. Taking into view what has now been communicated, and what was stated in my previous paper in regard to the existence of the older stratified rocks in the Silver Peak District, it will appear that Dr. Newberry’s generalizations were, in all probability, correct, and that we may expect to find in southern and southwestern Nevada the outcropping fossiliferous edges of the strata underlying the Carboniferous of the great Arizona or Colorado plateau, and that they will be proved to occupy an extensive area, and to yield a profusion of organic remains.