808 PROCEEDINGS OF THE CALIFORNIA 



faces of thin slabs of bluish-gray argillaceous limestones, and are crowded to- 

 gether 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 Wis- 

 consin Lead Region, around Big Bay des Noquets, and in many other locali- 

 ties in the country bordering on the Great Lakes. 



Both the upper aud 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 iu Wisconsin and Iowa. The. 

 particular period to which these lower Silurian forms may be referred is the 

 Trenton, including the Ckazy, 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 Plot Creek collection, namely : Brachiopods, Gasteropods^ 

 Cephalopods, Crinoids, Trilobites, and Corals; and there are among them sev. 

 eral of the most widely-distributed and most characteristic species of the 

 Lower Silurian. The following have been identified : Maclurca magna, a 

 characteristic Chazy species, and Pleurotoma lenticular is, Qrthis testudinaria and 

 Chatties h/coperdon, 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 Asa- 

 phus, which is represented by a species apparently new. There are also frag- 

 ments of crinoids or cyst ids closely resembling the species figured by Hall, in 

 the Palaeontology of New York, Vol. I, as Echino-encrinites anatiformis. . 



The rocks containing the above mentioned fossils crop out in the sides of a 

 deep canon ; 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, Heliolitcs spinipora and Synngopnra are recognizable; and 

 among the crinoidal fragmeuts 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 hund- 

 red 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. 



