XV.] CAMBRIAN AND SILURIAN IN NORTH AMERICA. 391 



New York (page 119), he however adopted a different view of 

 the Eed sand-rock, assigning it to the Calciferous ; and in 1855, 

 in his American Geology (II. 128), it was regarded as in part 

 Calciferous and in part Potsdam. In 1848, Professor C. B. 

 Adams, then director of the Geological Survey of Vermont, 

 argued strongly against these latter views, and maintained that 

 the Eed sand-rock directly overlaid the shales of the Hudson- 

 River group and corresponded to the Medina and Clinton for- 

 mations of the New York system. (Amer. Jour. Sci. (2), Y. 

 108.) He had before this time discovered in this sand-rock, 

 besides what he considered an Atrypa, abundant remains of a 

 trilobite, which Hall, in 1847, referred to the genus Conocephalus 

 (Conocoryphe), remarking at the same time that inasmuch as 

 this genus was (at that time) only described as occurring in 

 " gray wacke in Germany and elsewhere," no conclusions could 

 be drawn from these fossils as to the geological horizon of the 

 rocks in question. (Ibid. (2), XXXIII. 371.) In September, 

 1861, however, Mr. Billings, after an examination of the rocks 

 in question, pronounced in favor of the later opinion of Em- 

 mons, declaring the Red sand-rock near Highgate Springs, 

 Vermont, containing Conocephalus and Theca, to belong to the 

 base of the second fauna, " if not indeed a little lower," and 

 to be " somewhere near the horizon of the Potsdam." (Ibid. 

 (2), XXXII. 232.) 



The dark-colored fossiliferous shales which were asserted, 

 both by Adams and by Emmons, to underlie this Red sand- 

 rock, were, by the former, as we have seen, regarded as belong- 

 ing to the Hudson-River group, while by the latter they were 

 described as an upper member of the Taconic system ; which 

 was here declared to be unconformably overlaid by the Red 

 sand-rock, a member of the New York system. These slates, 

 a few years before, had afforded some trilobites, which, after 

 remaining in the hands of Professor Hall for two years or 

 more, were in 1859 described by him in the twelfth Report 

 of the Regents of the University of New York, as Olenus 

 Thompsoni and 0. Vermontana. He soon, however, found 

 them to constitute a distinct genus, for which he proposed the 



