to Section C {Geology). 457 



left behind no vestige of their existence? The explanation does 

 not lie in the nature of the sediments, which are not unfitted for the 

 preservation of fossils, nox 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 only plausible 

 supposition would appear to be that the organisms of that time had 

 not passed beyond the stage now represented by the larvae of 

 existing invertebrata, and consequently were either unprovided 

 with skeletons or at all events with skeletons durable enough for 

 preservation. If so, the history of the earlier stages of the evolution 

 of the invertebrata will receive no light from palaeontology ; and no 

 direct answer can be expected to the question whether, eighteen or 

 nineteen millions of years being taken as sufficient for the evolution 

 of the vertebrata, the remaining available eight millions would 

 provide for that of the invertebrate classes which are represented 

 in the lowest Cambrian deposits. On h priori grounds there would 

 appear to be no reason why it should not. If two millions of years 

 afforded time enough for the conversion of fish into amphibians, 

 a similar period should suffice for the evolution of trilobites from 

 annelids, or of annelids from trochospheres. The step from 

 gastrulas to trochospheres 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 extended those promising investigations which they are 

 only now laeginning to make into the rate of the variation of species. 



Unexpected Absence of Thermal Metamorphosis in Ancient Bocks. 



Two difficulties now remain for 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 

 v^rites 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 

 Eiver 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 began to form before 

 the end of the Keweenaw period, and consequently 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 



