160 Chemical and Geologicnl Essays of T. Sternj Hunt 



of the country — au important verification of the accuracy of his pre- 

 vious determinations. Great lithological changes are, however, ob- 

 served to take place in these localities, so distant from the best types 

 of the system ; thus, the "Ludlow and Wenlock" formations are no 

 longer distinctly separated by subordinate hpiestone, and are therefore 

 simply termed the "Upper Silurian" rocks, and these, changing their 

 soft argillaceous characters of "mudstone," become hard sandstones, 

 yet contam some well known organic remains; whilst the "Lower 

 Silurian" rocks, or Caradoc and Llandeilo formations, not only maintam 

 their usual fossil distinctions, but exhibit limestones of much greater 

 thickness than in any other part of their course. 



In brief, Mr. Murchison studied the rocks in England and Wales, 

 that repose beneath the Devonian, and from their fossil contents 

 separated them into groups, in the years 1831 and 1832, and published 

 his labors in that behalf in the spring of 1833. He continued his 

 study and labors through that year, and determined that the three 

 groups, which he had found next below the Devonian, were more 

 intimately connected together by their zoological remains than they 

 were Avith the next lower groups, which were likewise connected 

 toofether in the same manner. He here laid the foundation for the 

 division into upper and lower groups. He not only realized the im- 

 portance to science of his great discoveries, but his contemporaries like- 

 wise saw that his study of the organic remains had made it probable, 

 that his groups would be found extending over a great part of Europe, 

 and, possibly, determinable even in America. In 1835, in the fifth year 

 of his special labors on this formation, he applied to the whole of the 

 groups the name Silurian, and called the three upper groups the Upper 

 Silurian, and the remainder the Lower Silurian. Subsequently, paleon- 

 tologists found that his discoveries were world-wide in their application. 

 Fossils, that he described as characterizing the Upper Silurian and 

 Lower Silurian, w^ere found, or their equivalents, in Germany, Bohemia, 

 Canada and the United States, occupying the same relative position in 

 the rocky strata of the earth. His triumph was now complete. He 

 had brought order out of confusion, and determined the i-elative position 

 of miles of strata of the earth, founded upon the fossil contents which 

 had been hitherto unknown. 



In 1836, Mr. Sedgwick, the Cambridge professor, proposed the 

 name Cambrian, for a series of rocks, the mineralogy of which he had 

 been studying, and which he supposed to underlie the Lower Silu- 

 rian. But when the fossils came to be studied and described, they 

 were found to be Lower Silurian forms, and the strata merely an ex. 

 tension of the rocks described as Lower Silurian by Mr. Murchison. 



