330 C. D. WALCOTT — TIIK TERM "HUDSON RIVEB GROUP." 



In 1820,* Rev. Chester Dewey published an account of a section extending 

 from the Taconic mountains to the Hudson river at Troy. His observa- 

 tions and those of Eaton are too general in character to be ol' more than 

 historical interest at present. 



With the advent of the Geological Survey of Xew York, in 1836, system- 

 atic work was inaugurated and a classification developed which gave a 

 great impetus to geologic research in America. 



The first geologic district embraced the valley of the Hudson, and was 

 placed in charge of Dr. W. YV. Mather, who, in 1*40, proposed the name 

 " Hudson River Slate group." He says, in speaking of the rocks in the 

 valley of the Hudson : 



The lowest in the series is the Hudson River Slate group, consisting of slates, shales, 

 and grits, with intcrstratitied limestones, all of which occur under various modifica- 

 tions. This group is overlaid unconformably in many places by the various rock 

 formations of more recent origin. The next in order of superposition in the district 

 under examination * * * is the Shawangunk grits. * * * The next in order 

 is the Helderberg group; * * * and the Catskill Mountain group terminates 

 the series of indurated rocks in the First district. f 



From Kingston the Hudson River group ranges along the right or western 

 bank of the Hudson river to Albany, underlying the superincumbent rocks 

 unconformably, with few exceptions. A few fossil shells or impressions of 

 shells were found in the sandy beds, and some graptolites in the black duties 

 underlying the Shawangunk grits. In the final report of the firsl district, 

 Dr. Mather changed the name Hudson River Slate group to Hudson River 

 group.J The group as described may be classed by its structural relations 

 into two divisions: (1) The approximately horizontal, unaltered strata, w< at 

 of the line of disturbance in the valley of the Hudson. (2) The strata within 

 the area of disturbance in the immediate vicinity of the river and to the east 

 of the valley.§ 



The described sections of the undisturbed strata are portions ol' the highest 

 pari of the series, not far beneath the conformably superjacent Helderberg 

 division. A measured section of 141) feet 4 inches at Schoharie kill, Scho- 

 harie county, .-hows an alternating series of shales with arenaceous layers or 

 grit.-, Borne of which are calcareous. The thickness of the group could not 

 be ascertained in any part of the Hudson ami Champlain valleys, in conse- 

 quence of the rocks having been deranged, upheaved ami tilted ; but in the 

 valley- of Norman's kill, the Mohawk river ami Schoharie kill, they are 

 beautifully exposed to view. No actual measurement of these strata have 

 been made, loii it is estimated that they have a thickness of from 500 to 800 

 feet 1 1 The paleontologic evidence of the position of the strata consisted of 



• Amer. Jour. BoL, vol. j, 1820, pi 



nil, \ on Rep Qi irvey N. Y., i-i", p Ji-'. 



'..-., I. V V., '.■■•■I. !■ o-i I I. hi-!., I-I :, p. :'.'.7. 



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