146 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



volcanics can be found, as is not infrequently the case, it agrees with that of the Etcheminian. 

 It is thought therefore that those volcanics (the Coldbrook group) should be included in the 

 Basal Cambrian. 



Both in New Brunswick and in Cape Breton the Coldbrook group begins with lavas showing 

 deposition free of pressure, as they are^amygdaloidal; or with agglomerates devoid of evidence 

 of marked aqueous wear. The deposition therefore did not begin in deep water, or on exposed 

 sea coasts, or under heavy pressure. The foundation upon which the volcanics rest shows in 

 several places marks of deep subaerial decay at the line of contact. Calcareous bands are dis- 

 solved, leaving the siliceous portion of the strata. The feldspar of the granitic rocks is kaolinized 

 and the magnesian silicates are hydrated, impure graphite beds are changed to a black amor- 

 phous crumbling shale, and a depression or narrow valley is usually found at the contact of the 

 two terranes. These conditions appear to indicate that the pre-Cambrian complex had long 

 been above the sea level in these districts when the first Cambrian effusives were thrown out 

 upon it. 



Another point worthy of note in this connection is the large amount of feldspathic material 

 in the Etcheminian beds; the very sands are often composed of feldspathic grains, and these 

 largely of nonkaohnized feldspar, as though they had not been exposed to subaerial decay. 

 Feldspar in this condition is found in two kinds of deposits, those that are the result of glacial 

 wear and those found around volcanic vents, where particles of rock have .been torn from the 

 walls and blown out upon the surface of the earth. These if dropped into the sea would soon 

 be covered up by fine mud and preserved in their original crystaUine condition. The Etchemin- 

 ian appears to represent largely the submarine condition of such effusive rocks. On the other 

 hand, the Coldbrook series, as has been intimated above, represents the preceding subaerial 

 phase of the eruptives. It is true that we find in many places conglomerates at the contact of 

 these two series of rocks, thus diverse in appearance; but elsewhere there are no beds of rolled 

 fragments at the contact, and the passage is direct from ash beds or diabases to the slates and 

 sandstones. In reports of the Canadian Geological Survey of 1870-71, pages 57-59, etc., 

 relating to the province of New Brunswick, both these groups of rocks have been included in 

 the Huronian system. They may. be equivalent in age to the upper part of that series, but 

 unfortunately the absence of fossils in the original Huronian leaves this matter in doubt. 



As we contemplate the physical conditions of the initial epochs of Cambrian time in the 

 Maritime Provinces, we seem to see a region long elevated above the sea, now subjected to 

 depression nearly to the sea level, the depression being accompanied with extrusion of lavas 

 and volcanic mud and the ejection of stones and ashes. These at first were cast upon a land 

 surface, but, as the crust of the earth continued to sink, it was covered by the sounds and bays 

 of a shallow sea, diversified with pre-Cambrian ridges and islands, of greater or less extent. 



For the above reasons, as well as because the stratified rocks of the underlying complex 

 are markedly unconformable to the Cambrian, the volcanics are thought to belong to the latter, 

 and to give the natural base of this system. 



The accompanying table will then show the classification of the Cambrian system, as seen 

 in the Maritime Provinces of Canada. 



