SOME UNSOLVED PROBLEMS 29 



from trochospheres. The step from gastrulas to trocho- 

 spheres might be accomplished in another two millions, 

 and two millions more would take us from gastrulas 

 through morulas to protozoa. 



As things stand, biologists can have nothing to say 

 either for or against such a conclusion : they are not 

 at present in a position to offer independent evidence ; 

 nor can they hope to be so until they have vastly ex- 

 tended those promising investigations which they are 

 only now beginning to make into the rate of the 

 variation of species. 



Two difficulties now remain for discussion : one based 

 on theories of mountain chains, the other on the un- 

 altered state of some ancient sediments. The latter may 

 be taken first. Professor van Hise writes as follows 

 regarding the pre-Cambrian rocks of the Lake Superior 

 district: "The Penokee series furnishes an instructive 

 lesson as to the depth to which rocks may be buried 

 and yet remain but slightly affected by metamorphosis. 

 The series itself is 14,000 feet thick. It was covered 

 before being upturned with a great thickness of Keweenaw 

 rock. This series at the Montreal River is estimated to 

 be 50,000 feet thick. Adding to this the known thickness 

 of the Penokee series, we have a thickness of 64,000 

 feet. . . . The Penokee rocks were then buried to a 

 great depth, the exact amount depending upon their 

 horizon and upon the stage in Keweenaw time, when the 

 tilting and erosion, which brought them to the surface, 

 commenced. 



" That the synclinal trough of Lake Superior bogan to 

 form before the end of the Keweenaw period, and conse- 

 quently that the Penokee rocks were not buried under the 

 full succession, is more than probable. However, they 

 must have been buried to a great depth at least several 

 miles and thus subjected to high pressure and tempera- 



