THE ELASMOBRANCHS 59 



em Ireland. Throughout Norway and Sweden the older 

 Cambrian and Silurian formations were folded, over- 

 turned, and overthrust along individual fault planes for 

 distances as great as twenty to forty miles. Of this moun- 

 tain system, one of the greatest the world has ever seen, 

 the low and rounded Caledonian Mountains that com- 

 prise the Scottish Highlands are all that now remain. 

 Another range stretched across France, Germany, and 

 Austria, while still others were formed in northern Africa 

 and the Irkutsk basin of Siberia. 



The Devonian period, which followed this Caledonian 

 revolution, is identified for aU geologists with what in 

 the British Isles has long been called die 'Old Red Sand- 

 stone,' a geologic formation underlying the so-called 

 'Coal Measures' of the Carboniferous. In most parts of 

 the world the Devonian strata are of considerable depth, 

 and in England, where the geologist Sir Roderick 

 Murchison first studied the Old Red in Cornwall and 

 Devonshire, and renamed the system for the latter, they 

 reach a thickness of 10,000 to 12,000 feet; while in Aus- 

 traha the sedimentary and volcanic strata exceed 30,000 

 feet in depth and represent the most severe disturbance 

 that continent has ever experienced. 



For the marine invertebrates, the Devonian was a 

 heyday of evolution. Corals were building giant islands 

 in the warm seas, which extended at times up to the 

 Arctic Circle, and great bivalves, the forenmners of mod- 

 em clams and oysters, were in their ascendancy and com- 

 peting with snails, starfishes, sea urchins, squid, and cut- 

 tlefish. For the continental fishes, however, the Devonian 

 climate proved to be a mixed blessing. The Old Red of 

 Europe, as well as the Devonian strata elsewhere, is 

 primarily of continental origin and consists of conglom- 

 erates containing great river-eroded stones, sandstones 

 formed in the channels of streams, and siltstones and 

 shales originally deposited as mud in quiet waters and 

 frequently marked with cracks where successive layers 

 were dried by exposure in the alternation of extreme wet 



