MODERN GEOLOGY 



transition rocks into chronological groups, since famil- 

 iar 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 Devon- 

 shire, England. It was subsequently discovered that 

 these systems of strata, which crop out from beneath 

 newer rocks in restricted areas in Britain, are spread 

 out into broad, undisturbed sheets over thousands of 

 miles in continental Europe and in America. Later on 

 Murchison studied them in Russia, and described them, 

 conjointly with Verneuil and Von Kerserling, in a pon- 

 derous and classical work. In America they were 

 studied by Hall, Newberry, Whitney, Dana, Whitfield, 

 and other pioneer geologists, who all but anticipated 

 their English contemporaries. 



The rocks that are of still older formation than those 

 studied by Murchison and Sedgwick (corresponding in 

 location to the "primary" rocks of Werner's concep- 

 tion) are the surface feature of vast areas in Canada, 

 and were first prominently studied there by William I. 

 Logan, of the Canadian Government Survey, as early as 

 1846, and later on by Sir William Dawson. These rocks 

 comprising the Lauren tian system were formerly 

 supposed to represent parts of the original crust of the 

 earth, formed on first cooling from a molten state ; but 

 they are now more generally regarded as once-stratified 

 deposits metamorphosed by the action of heat. 



Whether "primitive" or metamorphic, however, 

 these Canadian rocks, and analogous ones beneath the 

 fossiliferous strata of other countries, are the oldest 

 portions of the earth's crust of which geology has any 



157 



