THE LOWER AKU UPPER SILURIAN AGES. 59 



thick, but in Russia, it is less than 1,000 feet. In 

 the eastern part of America the thickness is estimated 

 at quite as great an amount as in Europe, while in the 

 region of the Mississippi the Silurian rocks are scarcely 

 thicker than in Russia, and consist in great part of 

 limestones and fine sediments, the sandstones and 

 conglomerates thinning out rapidly eastward of the 

 Appalachian Mountains. 



In both plateaus the earlier period of coarse accu- 

 mulations was succeeded by one in which was clear 

 wa*er depositing little earthy sediment, and this 

 usually fine ; and in which the sea swarmed with 

 animal life, from the debris of which enormous beds 

 of limestone were formed the Trenton limestone of 

 America and the Bala limestone of Europe. The 

 fossils of this part of the series open up to us the 

 head- quarters of Lower Silurian life, the second great 

 fauna of Barrande, that of the Upper Cambrian of 

 Sedgwick; and in America more especially, the 

 Trenton and its associated limestones can be traced 

 over forty degrees of longitude ; and throughout the 

 whole of this space its principal beds are composed 

 entirely of comminuted corals, shells, and crinoids, 

 and studded with organisms of the same kinds still re- 

 taining their forms. Out of these seas, in the Euro- 

 pean area, arose in places volcanic isletfj, like those oi 

 the modern Pacific. 



In the next succeeding era the clear waters became 

 again invaded with muddy and sandy sediments, in 

 various alternations, and with occasional bands of lime- 



