306 EVOLUTIONAL GEOLOGY. 



immediatel}' underljdng the Cambrian, these have left behind no ves- 

 tige of their existence? The explanation does not lie in the nature of 

 the sediments, which are not unlitted for the preservation of fossils, 

 nor in the composition of the then existing sea water, which may have 

 contained quite as much calcium carbonate as occurs in our present 

 oceans; and the onh" plausible supposition would appear to be that 

 the organisms of that time had not passed beyond the stage now rep- 

 resented by the larva? of existing invertebrata, and consequently were 

 either unprovided with skeletons, or at all events with skeletons dura- 

 ble enough for preservation. If so, the history of the earlier stages 

 of the evolution of the invertebrata will receive no light from 

 paleontology; and no direct answer can be expected to the question 

 whether, eighteen or nineteen millions of 3'ears being taken as suffi- 

 cient for the evolution of the vertebrata, the remaining available eight 

 millions M^ould provide for that of tiie inA^ertebi'ate classes whicli are 

 represented in the lowest Cambrian deposits. On a priori grounds 

 there would appear to be no reason why it should not. If two mil- 

 lions of yetirs afforded time enough for the conversion of lish into 

 amphibians, a similar period should suffice for the evolution of trilo- 

 bites from annelids, or of annelids from trochospheres. The step 

 from gastrulas to trochospheres might })e 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 

 vastl}^ extended those promising investigations which they are only 

 now beginning to make into the rate of the variation of species. 



UNEXPECTED ABSENCE OF THERMAL METAMORPHOSIS IN ANCIENT ROCKS. 



Two difficulties now remain ^or discussion — one based on theories of 

 mountain chains, the other on the unaltered 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, where the tilting and 

 erosion, which brought them to the surface, commenced. 



"That the synclinal trough of Lake Superior began to form before 

 the end of the Keweenaw period, and consequently that the Penokee 



