BIGSBY PALAEOZOIC BASIN OF NEW YORK. 435 



3. The same species of fossils are, as a consequence, more widely 

 disseminated in this stage. 



4. Species of fossils are few, and individuals are many in the early 

 part of the lower stage ; but after the discontinuity of life at the close 

 of Calciferous Sandstone, which was much more complete than that 

 at the close of the Hudson-River Rocks, animated beings peopled the 

 Lower Silurian seas in greater numbers perhaps than in the upper 

 stage ; but upon the whole (for there are exceptions) their forms 

 are plainer and simpler. 



The Middle Stage. — Many practical advantages are obtained by 

 admitting the existence of a middle stage, although there may be 

 difference of opinion as to its upper termination and other points. 



In this place I simply express my belief that the change of con- 

 ditions, or break, which created this middle or transitional stage, did 

 not proceed from a great crust-rupture in the region now called the 

 United States, nor from any disturbance at all comparable to the 

 post-carboniferous uplift, but perhaps from violent currents carrying 

 westwards the products of distant catastrophes, such currents being 

 impelled by a crust-undulation. The further discussion of this sub- 

 ject I postpone to a more convenient part of this paper. 



The middle stage commences with the Oneida Conglomerate of 

 eastern New York and Lower Canada, and it may be made to end 

 with the Clinton Rocks. 



The fact is, that in eastern and central New York the Clinton 

 Rocks have intimate and extensive relations with Medina Sandstone* 

 (still closer in Pennsylvania according to H. D. Rogers), while in 

 the western portion of the State and in its upper beds it has slowly 

 graduated into the Niagara section, the great mass, however, for some 

 hundred square-miles, being arenaceous, and in all respects having 

 a transitional character. 



The middle stage of the Silurian system in the State of New York 

 is firmly based upon the great and frequent changes of mineral cha- 

 racter which it presents, and upon the notable disappearance of Lower 

 Silurian races, accompanied by the introduction of a few important 

 and new forms of animal and vegetable life. 



It is remarkable for the great predominance, first, of conglomerate 

 and grit, and, secondly, of sandstones, in the higher and western 

 beds, diversified almost ad infinitum by intermixtures of clay, lime, 

 and iron — no stratum remaining the same even for a moderate breadth 

 of country. Further, everywhere in this stage a gradual change was 

 always going on in a westerly direction, from coarseness to fineness — 

 from quartz-shingle through clay to limestone, with all the usually 

 concomitant succession of new organisms which takes place in existing 

 sea-beds. 



* Hall (Palaeontology, &c. ii. p. 2) says that, in the western portion of the 

 State, the limit between the Medina Sandstone and the Clinton group is well 

 defined, and the materials distinct ; but in the central part of the State, the 

 Clinton (like the Medina Sandstone) begins by a shaly deposit, succeeded by 

 alternations of sandstone, in chai*acter like those of Medina Sandstone. 



