BIGSBY PALEOZOIC EOCKS OP NEW YOBJS:. 255 



Conglomerates and grits form the basement of the greater divisions, 

 and mark the transitions (see Potsdam Sandstone, Oneida Conglo- 

 merate, Oriskany Sandstone, and Catskill Group). 5. The great 

 similarity between Hudson-River Eocks and Oneida Conglomerate 

 is thus well brought out, though in supposed different stages. Lo- 

 cally they are sometimes undistinguishable. 6. The middle or trans- 

 itional Silurian stage, as well as the section immediately below, 

 exhibits numerous and great mineral changes and complications, 

 such as are indicative of frequent movement. The Hudson-River 

 Rocks contain eight varieties of sediments ; the Medina Sandstone 

 contains five ; Clinton Rocks, ten ; besides the minuter alterations, 

 which it would be profitless to describe. 7. Excepting in the 

 Middle Silurian stage, lithological differences on the same horizon 

 are moderate in extent, although many and sometimes occupying 

 large spaces. 8. The arenaceous and argillaceous strata (usually 

 near old coast-lines) are more changeable than the calcareous, the 

 latter being formed under quieter conditions. 



Our Synoptical View * pouited out, as a determinate law, that, 

 with rare exceptions, these sediments graduate into each other, just 

 as we see in Wales, in parts of America, Scandinavia, and Russia. 

 Although this change takes place slowly and tranquilly, the majority 

 of the organic beings belonging to the terminated section perish. 

 We are thus taught that destruction of life can take place without 

 the accompaniment of a marked disturbing force. 



The sedimentary deposits of Wales and the adjacent English 

 counties only agree to a certain extent with those of New York. 

 When beyond the metamorphic action of the Appalachian crust- 

 movement, the latter are little affected in their mineral condition 

 by infiuences acting from below. The shales are soft and shivery, 

 the sandstones of moderate hardness, and the limestones are dark- 

 coloured, fine-granular, or crystalUne (fossiliferous), and in abruptly 

 separated beds. It is often remarked that fossHiferous beds of cry- 

 stalline limestone are intercalated with others of opake granular 

 structure, the separating planes being perfectly abrupt. The cry- 

 stalline form cannot here have been derived from metamorphism, 

 because, if so, it would have affected the strata beneath. The sup- 

 position of great purity in the crystalline rock will not help us, be- 

 cause it is well known that chemical precipitation throws down pure 

 carbonate of lime in the opake amorphous form (Carpenter). The 

 points of difference between the two kinds of beds are these : the 

 amorphous beds are very dark-coloured, often fetid, and have few or 

 no fossils; whereas the crystalline are pale-grey, are made up of 

 crushed fossils, and present rhomboidal facets. The cleavages are 

 often rare and obscure. This is not the condition of the sediment- 

 ary rocks of Wales. Metamorphic action has been in frequent play 

 among its mountain- masses. We find here (as, under similar cir- 

 cumstances, in Bohemia, according to Barrande) an abundance of 

 quartzites, micaceous sandstone, flagstones, roofing-slates, distorted 

 shells, and much powdery chlorite. Thirty forms of sedimentary 

 ^' Quart. Journ. Geol. Soc. vol. xir. p. 335. 



