ORDOVICIAN FAUNAS IN LAKE CHAMPLAIN VALLEY 455 



well exposed, seems to be a very constant and characteristic layer of fine grained 

 sandstone or quartzite. It is particularly well shown at Valcour and Crown point,* 

 and seems to indicate a retreating sea at the close of the period immediately pre- 

 ceding the deposition of the Black River strata. The succeeding Black River beds 

 are of heavy bedded black limestone having thin shaly partings, and ordinarily in 

 the upper portion of each distinct thick layer a fossiliferous zone. 



The cliange to beds carrying a true Trenton fauna is usually abrupt, the latter 

 formation consisting of much more thinly bedded and usually softer black lime- 

 stones and slates. A great variation in the character of these beds is observable, 

 and in consequence the limits of particular conditions which favored the develop- 

 ment of particular species is very much more marked than in the Trenton Falls 

 paleontologic province, which was studied by the writer in connection with the 

 present investigation. f In the Champlain Trenton, beds of tough and almost 

 barren black limestone, or similar limestones containing fossils preserved almost 

 entire, are succeeded by far softer beds in which all the contained fossils are en- 

 tirely fragmentary. 



The frequent lenticular character of the limestones is a marked characteristic. 

 These lenses usually consist of a gray, finely crystalline, almost saccharoidal, lime- 

 stone, usually very fossiliferous, within layers of black slate. The rich fauna of 

 these granular layers is no doubt due to more numerous currents, and certain species, 

 such as Zygospira exigua and Z. recurvirostris, Teiradiumfibratum, PlaiysLrophia hiforata, 

 Bucania punctifrons, Conradella compressa, Holopea paludiniforniis, Baihyurus spiniger, 

 and Ceraurus phurexmdliemus, seem to be almost wholly confined to deposits of this 

 kind. 



On the other hand, a large assemblage of linguloid species, Parasirophia hemi- 

 plicata, most of the lamellibi'anchs (as might be expected), Frotowartlda cancellata, 

 Isotelus gigas, and Trinudeus conceniricus, were almost wholly denizens of the mud 

 and are found principally in the shaly portions of the rock. Toward the upper 

 portion of the series, the changes of the sea bottom evidently became a great deal 

 more rapid and increasingly shaly layers appear, with attenuated fine crystalline 

 lenses between. 



In several instances, notably on Crown point (Fort Frederick), occur thin, tough, 

 black, fine grained, highly carbonaceous layers, which appear to be the consoli- 

 dated black mud of perhaps decaying organic remains. The upper surfaces of such 

 layers are highly polished, but this may either have been the result of wear before 

 subsequent deposition took place, or the result of the slip or " slickensides " from 

 movement of the overlying layers on the more slippery surface. Such layers are 

 constant over considerable areas. Many layers, especially those of gray crystal- 

 line limestone, are conglomeratic, containing rounded or elongated pebbles and 

 masses of what was evidently thick black mud worn from neighboring land sur- 

 faces and entombed. Similar intraformational conglomerates are noted from the 

 Galena series of Minnesota, f 



*J. D. Dana: Discoveries of Reverend A. Wing; Am. Jour. Sci., 3d ser., vol. xiii, p. 415. E. 

 Brainerd and H. M. Seely : Bull. Am. Mus. Nat. Hist., vol. viii, p. 315. 



fSee T. G. White: Trans. New York Acad. Sei., vol. xv, 189G, pp. 71-96, and Am. Jour. Sci., 3d 

 ser., vol. ii, 1896, pp. 430-432 ; also C. S. Prosser and E. R. Cummings, Fifteenth Ann. Rept. New 

 York State Geologist, 1898, pp. 615-659. 



X F. W. Sardeson : American Geologist, vol. xxii, 1898, pp. 315-323. 



