REPORT OF TEE CHIEF ASTRONOMER 645 



SESSIONAL PAPER No. 25a 



The present chapter contains a discussion of various tests of the suggested 

 hypothesis. These include, first, the witness of laboratory experiments; secondly, 

 the testimony of the Black sea — a basin where modern limestone is being 

 deposited by the organic alkali because of the lack of a scavenging system over 

 most of the basin-floor ; thirdly, the evidence based on the chemistry of the rivers 

 draining pre-Cambrian terranes; and, fourthly, the lithological evidence of pre- 

 Cambrian sedimentary deposits. In particular, the testimony of the microscope 

 to the chemical origin of thick Cambrian and pre-Cambrian limestones is out- 

 lined, and the systematic chemical variation of the limestones through geologi- 

 cal time is quantitatively discussed. 



This chapter is chiefly a composite reprint of the preliminary papers. The 

 more complete presentation of the hypothesis may be of some value to those who 

 have to do with the paleontology or chemical geology of the pre-Silurian forma- 

 tions. For himself the writer has found more satisfaction in this explanation 

 of the dearth of fossils and in the correlative hypothesis of the dolomites, than 

 in any of the older views on these problems. 



Explanations of the Unfossiliferous Character of pre-Cambrian Sediments. 



1. Hypothesis of the metamorphic destruction of fossil remains. — The view 

 that shells or skeletons were actually once present in anything like the propor- 

 tions characteristic of Silurian or later marine sediments, and have since been 

 destroyed through either static or dynamic metamorphism, has proved as unsatis- 

 tory for these pre-Silurian American terranes as it has for pre-Silurian terranes 

 throughout the world. The opposed hypothesis that the hard parts of marine 

 animals were seldom entombed in pre-Cambrian strata is worthy of careful 

 examination. This latter hypothesis is multiple, since it may postulate 

 different causes for the lack of entombment. All postulates must, however, 

 recognize the fact that the mechanical conditions of burial and preservation 

 were all present. So far as chemical composition, detrital composition, rapidity 

 of deposition, etc., are concerned, the sediments of the Cordilleran province, as 

 of other pre-Cambrian formations, are ideal for perfect fossilization. 



2. Brooks hypothesis. — The admirable essay of W. K. Brooks in the Journal 

 of Geology (Vol. 2, 1894, p. 455), states one conceivable hypothesis. He suggests 

 that the photobathic zone of the sea, where it reached to the bottom, first became 

 inhabited just before Cambrian time. He considers it probable that all the 

 fundamental types of animals from protozoon to mollusc and arthropod, but all 

 as yet soft-bodied, had been evolved in the surface waters of the open sea, far 

 from land. At the close of pre-Paleozoic time the pelagic fauna first discovered 

 the advantages of life alongshore and the special advantages of life on the bot- 

 tom of the shallow coast-waters. Owing to the intense struggle for existence 

 within the shore zone, there was, in early Cambrian time, a rapid acceleration 

 of development which tended towards the relatively sudden evolution of hard 

 calcareous and chitinous structures, which functioned as means of protection, 

 of offence, or of otherwise perfecting the animals for successful combat. The 



25a — vol. iii — 42^- 



