8 STBATIGKAPHICAL GEOLOaT. 



Streptorhynchus sinuata has a vertical range of only a few feet below the 

 middle of the Group, and S. sulcata has a vertical range of only a few feet near 

 the upper part of the Group. 



Orthis hellula, 0. plicatella, 0. Jissicosta, 0. ella and Cythere cincinnatiensis 

 are confined within a vertical range of about 100 feet, near the middle of the 

 Group. O. pUcatella, O. Iriplicatella, 0. Jissicosta and O. jamesi vary much 

 in size and proportional length and breadth and general appearance, and some- 

 times run so close together that it is only by close observation that the species 

 are separated. Orthis ella varies so much in size and number of plications that 

 it could be separated into three forms sufficiently distinct to have specific names, 

 if the forms were found only in distinct Groups of rocks. But, probably, no 

 shell indicates the unceasing change and development of animal life during the 

 deposition of the Group as much as the Orthis lynx. It is found of all dimen- 

 sions, from 1-16 of an inch to 2 inches in length, breadth and thickness. The 

 mesial sinus is usually occupied with three plications, and the mesial fold with 

 four; but sometimes the mesial sinus has only two plications, and sometimes it 

 has four and even five, while the mesial fold always contains one more than the 

 sinus, if the specimen is regularly developed. The more profound the sinus, the 

 fewer plications in it. Some specimens are much longer than they are wide; 

 others much wider than long. Some specimens, with hinge-line shorter than the 

 width of the shell, become globose and nearly as round as an apple ; others have 

 the hinge-line prolonged to double the width of the shell, and have nearly the 

 form of Spirifera mucronata. Small specimens of the globose form are marked 

 with about sixteen plications, while the long-eared forms have as many as forty 

 on each valve. Some specimens have thin shells ; while others, no larger, have 

 very thick ones. These extreme varieties do not occupy the same layers of rock, 

 but different strata. Considerable variation exists, however, in specimens occu- 

 pying the same layer; and so many intermediate forms are found in different 

 layers, that the extremes in the Group are linked together. 



The maximum thickness of the lower Silurian, as shown bj' the Groups men- 

 tioned, is 48,745 feet; and the fossiliferous part of the metamorphic rocks, 36,- 

 500 feet; making a total of 85,245 feet, or a little over sixteen miles from the 

 top of the Hudson Eiver Group to the base of the fossiliferous rocks. In other 

 words, if all the Metamorphic and Lower Silurian Groups were fully represented, 

 at their greatest thickness, on the Hudson River or at Cincinnati, we would ex- 

 pect to find fossils by digging or boring at these places for sixteen mijes. The 

 fact is probable, however, that part of the earth was dry land, while another part 

 was covered with an ocean; and that the dry land was worn away by the action 

 of rain, and other causes, while the ocean bed, gradually filled up, as the Atlantic 

 fills to-day, by sedimentary deposition. There is no evidence of dry land, during 

 all this period ; but the negative evidence to the contrary, in the total absence of 

 land plants and animals. Dry land may have existed, however, in the shape of 

 barren rocks and disintegrated matter, for mechanical deposition; but if it did 

 not exist, it is presumed that deposition took place more rapidly at the bed of 

 the ocean at one place than at another, and that the ocean currents removed 

 what had been deposited at one place, and carried it to another ; so that, in either 

 case, the maximum thickness of each Group is the measure of the lapse of time 



