THE STORY OF NINETEENTH-CENTURY SCIENCE 



this system, based mainly on the fossils. Meantime it 

 was found that, judged by the fossils, the strata that 

 Brongniart and Cuvier studied near Paris were of a still 

 more recent period (presumed at first to be due to the 

 latest deluge), which came to be spoken of as tertiary. 

 It was in these beds, some of which seemed to have been 

 formed in fresh-water lakes, that many of the strange 

 mammals which Cuvier first described were found. 



But the " transition " rocks, underlying the " second- 

 ary " system that Smith studied, were still practically 

 unexplored when, along in the thirties, they were taken 

 in hand by Roderick Impey Murchison, the reformed 

 fox-hunter and ex-captain who had turned geologist to 

 such notable advantage, and Adam Sedgwick, the brill- 

 iant Woodwardian professor at Cambridge. 



Working together, these two friends classified the 

 transition rocks into chronological groups, since familiar 

 to every one in the larger outlines as the Silurian system 

 (age of invertebrates) and the Devonian system (age of 

 fishes) names derived respectively from the country of 

 the ancient Silures, in Wales, and Devonshire, England. 

 It was subsequently discovered that these systems of 

 strata, which crop out from beneath newer rocks in re- 

 stricted areas in Britain, are spread out into broad un- 

 disturbed sheets over thousands of miles in continental 

 Europe and in America. Later on Murchison studied 

 them in Russia, and described them, conjointly with 

 Yerneuil and von Kerserling, in a ponderous and classi- 

 cal work. In America they were studied by Hall, New- 

 berry, Whitney, Dana, Whitfield, and other pioneer 

 geologists, who all but anticipated their English contem- 

 poraries. 



The rocks that are of still older formation than those 



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