26 FROM FISH TO PHILOSOPHER 



the time of their evolution. Some have assigned this to 

 the Cambrian and some to the Ordovician period— the 

 former is accepted here. The Cambrian was ushered in 

 by the Chamian revolution, which in North America 

 took the form of what American geologists call 'the 

 Grand Canyon disturbance/ Where the Colorado River 

 has cut its gorge for 200 miles through Arizona, there 

 had been deposited a series of pre-Cambrian sedimen- 

 tary rocks (the Grand Canyon System) composed of 

 conglomerates, limestones, shales, and quartzites, the 

 whole some 12,000 feet in thickness; during the Grand 

 Canyon disturbance these strata were uphfted and 

 broken by faults into mountains two miles high, moim- 

 tains which were almost entirely reduced to sea level 

 again shordy after the erogenic disturbance passed. 

 Thereafter the Cambrian as a whole was one of conti- 

 nental submergence, and before its close more than 30 

 per cent of North America (to mention a well-studied 

 area) was covered by a sea that invaded the continent 

 in two great troughs, the Cordilleran trough extending 

 from Alaska to the Gulf of California and submerging 

 the area now represented by the Rocky Mountains, and 

 the Appalachian trough extending from Newfoundland 

 to the Gulf of Mexico and submerging the area roughly 

 corresponding to the later Appalachian Mountains. (His- 

 torical geologists are sometimes confusing by their 

 choice of names, which cannot simultaneously locate an 

 area in two dimensions of geography and the third di- 

 mension of time.) 



The protovertebrate, with its spindle-shaped body 

 and segmentally arranged muscles adapted to rhythmic 

 contractions, is such, as Chamberlain suggested, as 

 would be evolved in response to the motion of rivers 

 flowing constantly in one direction, rather than in the 

 sea where local motion, if any, is a gentle ebb and flow. 

 No similar form has ever been evolved in the sea. It is 

 consonant with the evidence to suppose that the ances- 



