C. D. Walcott — The Taconic System of Emmons. 315 



In commenting upon Professor Marcou's reference of the 

 Potsdam sandstone to the " Taconic System," he objects to 

 such references on stratigraphic grounds, as is shown by his 

 letter of January 28th, 1861. 



These later letters of Dr. Emmons prove that he considered 

 the " Taconic System " to include the Huronian of Logan and 

 the graptolite-bearing shales of the Hudson valley, from his 

 letter of November 2*0th, 1860, he also included the Para- 

 doxides beds of the " Upper Taconic " which equal the Pri- 

 mordial group of Barrande, which "is only Lower Silurian," 

 and declared that " the Lower Silurian is strictly unconform- 

 able to every part of my Taconic series." 



Despite the statements made in the preceding paragraph, 

 I think we may say that Dr. Emmons regarded the original 

 " Taconic System" as stratigraphically unconformable and sub- 

 jacent to the Potsdam sandstone of the Lower Silurian of the 

 New York section and believed it to rest unconformably upon 

 the crystalline gneiss at its base and to form a great system of 

 sedimentary rocks between the gneiss and Potsdam sandstone. 



Comparison and Discussion. 



Comparison. — A comparison of the geology of the Taconic 

 area as known at the present time with the geology of the 

 same area as known to Dr. Emmons develops several points 

 of agreement. His lithologic descriptions are usually easily 

 verified ; and the general dip and arrangement of the strata 

 within the " Taconic System" are the same with the exception 

 of the relations of the strata referred to the "Lower" and 

 " Upper Taconic." 



The points of disagreement are: the identification of the 

 geologic age of the formations of the " Lower Taconic ;" the 

 stratigraphic relations of the " Lower " and " Upper Taconic ;" 

 the stratigraphic relations of the "Upper Taconic" and the 

 superjacent Silurian formations, and the value of the strati- 

 graphic and paleontologic identifications of the age of the 

 " Upper Taconic " slates. 



1. Dr. Emmons considered the " Lower Taconic " to be 

 composed of three non-fossiliferous pre-Silurian formations — 

 " Granular quartz, Stockbridge limestone and Talcose slates" 

 (see fig. 10) that were unconformably superjacent to the crystal- 

 line gneisses beneath and conformably subjacent to a great 

 series of slates, forming the " Upper Taconic," that, in turn, 

 were unconformably subjacent to the lowest of the Lower 

 Silurian formations, the Potsdam sandstone. 



"We now know that the base of the " Taconic System," the 

 " Granular Quartz," contains fossils that prove it to be the geo- 

 logic equivalent of the greater portion of the " Upper Ta- 



