18 PALEONTOLOGY OF KENTUCKY. 



and the Hudson River groups are of main importance to Kentucky geologists. 

 These two cover extensive areas of our State, and show in some places con- 

 siderable thickness. The balance are either wanting in Kentucky altogether, 

 or exposed in only very few and very limited spots. 



The Hudson River Group is generally known as the Cincinnati Group, on 

 account of its excellent exposure at and around Cincinnati. In Kentucky, the 

 blue limestone of the Trenton forms the surface-rock of that rich and world- 

 renowned district known as the "Blue-grass Country," and is, there, the 

 source of the most productive soil of the United States. In consequence of 

 the rapid decomposition of this rock, when exposed to the influence of sun- 

 shine and rain, it keeps the soil in an everlasting virginity, a soil which, a 

 thousand years from to-day, will produce as well as it does at present, pro- 

 vided all other conditions remain the same. The Upper Silurian is divided 

 into the following groups : 



1. The Oneida Conglomerate ; 2. The Medina Sandstone ; 3. The Clinton 

 Group ; 4. The Niagara Group ; 5. The Onondaga Salt Group ; 6. The Lower 

 Helderberg Group. Of these subdivisions of the Upper Silurian, only the 

 Clinton and the Niagara groups deserve especial notice as Kentucky forma- 

 tions. The Clinton Group is only found in few and isolated places, and never 

 attains a thickness of any consideration. The Niagara Group embraces the 

 same kind of rocks which form the bed of the Niagara river at its world-re- 

 nowned falls, whence it derives its name. In Kentucky it neither covers 

 extensive areas nor attains great thickness ; but it furnishes, in some places, 

 excellent building stone. Near Louisville, we find it well exposed in the 

 quarries east of the city, providing Louisville with most of the limestone 

 required for building purposes. Here the Niagara rocks are very rich in fos- 

 sils, which are mostly well preserved, and have furnished a great deal of the 

 material now in the valuable collections of several Louisville geologists, and 

 which enables science to acquire an extensive knowledge of the fauna of the 

 Niagara period. 



DEVONIAN FORMATION. This formation is named by Murchison after Devon- 

 shire, in England, where it is prominently represented in the surface-rocks. 

 Here in America we have subdivided it in the following groups : 



1. Oriskany Sandstone ; 2. Upper Helderberg Group ; 3. Hamilton Group ; 

 4. Portage Group ; 5. Chemung Group; and 6. Catskill Group. 



The Oriskany sandstone is placed here at the base of the Devonian column, 

 whilst others consider it as the youngest member of the Silurian groups. It is 

 extremely difficult to decide which of the two places assigned by different 

 geologists to the Oriskany sandstone, is the proper and correct one. If, in 

 accordance with the opinion of the older geologists, the Silurian period had 



