256 PALAEOZOIC TIME — UPPER SILURIAN. 



Here, then, a new phase in American geography is brought out to 

 view. The Lower Helderberg limestones are mainly Appalachian 

 formations ; for even the New York part is directly in the range 

 of the Appalachians of Pennsylvania. It is worthy of note that 

 this limestone formation of the last period of the Upper Silurian 

 is the first one that was produced over the Appalachian region 

 after the Trenton in the middle of the Lower Silurian. But the 

 Trenton beds spread over both East and West, while the Lower 

 Helderberg occur only sparingly in the West ; and in this the two 

 periods are in contrast, the older formation having the widest dis- 

 tribution. 



The sinking of eastern New York which was required for the 

 formation of the Lower Helderberg limestones probably submerged, 

 wholly or in part, the Greent Mountain region, which appears to 

 have been dry land in the Niagara period. That it affected the rest 

 of the Appalachian region to the southwest is manifest from the dis- 

 tribution of the rocks in that direction. Outside of the limestone- 

 making seas in face of the Atlantic waves there were probably bar- 

 riers of sand thrown up by the sea, or of emerged land, repeating 

 in this respect the condition in the Trenton period. It is true that 

 limestone reefs may and do form on an exposed coast ; but only in 

 case the submerged bottom of the continental border (which at this 

 time was over 100 miles wide) was not so near the surface that its 

 sands or mud could be thrown over the growing reefs by the waves 

 or currents. Limestones have been made for the most part from 

 the relics of species that require clear water, like the coral lime- 

 stones of existing seas. So great a breadth of shallow border as that 

 of the American coast could hardly have existed at the time with- 

 out sand-reefs being thrown up to fend off the waves, unless the 

 whole were deeply submerged. 



OBSERVATIONS ON THE AMERICAN UPPER SILURIAN. 



General features. — Fresh-water lakes and rivers, fresh-water de- 

 posits, and land or fresh-water animal life, continue unknown 

 through the American records of the Upper Silurian, as thus far 

 investigated. Such rivers and lakes probably existed, as it is cer- 

 tain there was dry land ; but they have left nothing that survived 

 subsequent changes. It is barely possible that some of the Mol- 

 lusks may have lived in fresh waters ; but the remains are so mingled 

 with species that are obviously salt-water types that it cannot be 

 proved to be true of any. 



With regard to land-plants there is some doubt. Certain sub- 



