J. D. Dana — Brief history of Taconic ideas. 417 



This system of 1855, like that of 1844, has a top and bot- 

 tom of Cambrian rocks. The succession of rocks in the 

 " Lower" Taconic— which, it should be remembered, were the 

 only rocks in the system when Rogers wrote in 1841 and 1844 

 and Mather in 1848 — coincides with the views early set forth 

 by Rogers, and with the order established by the most recent 

 discoveries of fossils. 



Professor Emmons, on page TO of the American Geology, 

 earnestly sustains the azoic character of the " Lower " Ta- 

 conic, and particularly that of the slates of Saddle Mountain 

 or Greylock. But the sentences about the occurrence of fossils 

 in other parts of the " Lower " Taconic are left ambiguous. 

 Speaking of the Taconic system as a whole on the closing page 

 (p. 122) he says : (3) " It is a vital system, having been deposited 

 during the period when organisms existed." (6) It " carries us 

 back many stages farther in time when life gave vitality to its 

 waters than the Silurian." He also says : " (1) Its series, 

 divided into groups, are physically unlike the Lower Silurian 

 series." " (2) It supports unconf ormably at numerous places 

 the Lower Silurian rocks." Again on page 49 : This group, 

 " the ' Lower ' Taconic, is mostly anterior to the Organic 

 period." No locality of Taconic trilobites is mentioned in the 

 chapters except that of Bald Mountain in Rensselaer Co., 

 !N. Y., and one of much interest in Augusta Co., Virginia, 

 which had afforded Professor Emmons the small species he 

 named Microdiscus qua-dricostaftus. 



In 1854 or 1855 new Trilobites, related to those of Bald 

 Mountain, were found in the Black Slates of West Georgia, 

 Vermont, within the range of beds referred to the " Upper " 

 Taconic. Passing into the hands of Mr. Zadock Thompson, 

 who had been assistant in the Geological Survey of the State 

 under Professor C. B. Adams, the specimens were sent to 

 Professor Hall, and in 1859, they were figured and described 

 by him as " Trilobites of the shales of the Hudson River 

 Group," under the names Olenus Thompsoni, 0. Yermontana 

 and Peltura holopyga, and the beds were thus made equiva- 

 lents of the Bald Mountain slates.* Although these fossils 



*J. Hall, Twelfth Annual Rep. N. Y. State Cab. Nat. Hist., pp. 59-62, 1859. 

 Also Yol. Ill, N. T. Paleontology, 1859, p. 325; Rep. Geol. Vermont, p. 367, 

 where the two species of Olenus are united under the name Barrandia Vermon- 

 tana. In Vol. Ill of the Paleontology, Professor Hall says, on page 94, ''In the 

 western flank of the Green Mountain range, the great variety of schists, desig- 

 nated as talcous, mica, gneissoid mica, hornblende and calcareous mica slates are 

 all results of metarrorphism of Silurian strata ;" and on p. 83 : " It is now many 

 years since the belt of the country was regarded as one great Primary mass. 

 Later observers began to yield a little and contented themselves with a Primary 

 axis; and now we have the evidence, derived from fossils occurring at intervals 

 over much of the area between the Hudson and the Connecticut rivers, as well as 



