560 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



crYstalline basaltic rocks, or the gabbros, which, on the "theoretical 

 belief" that they were stratified, and on account of their cutting across 

 the strike of some of the limestone belts — as any eruptive mass natu- 

 rally would — were placed in a system newer than the Laurent ian 

 proper. Subsequent investigations have shown that these gabbros are 

 — in part, at least — interbanded with tlie Laurentian limestones, so 

 that they must be of the same age as those limestones, or else later in- 

 trusions. That they are eruptive rocks is the testimony of the present 

 director of the Canadian Survey, and is in accord with the investigations 

 of the best petrographers the world over ; while our own investigations 

 in the Adirondacks have led us to similar conclusions. Moreover they 

 have been found by us occurring in dikes in the granite of Eastern 

 Massachusetts. 



Further, the Hastings series, although at first looked upon as con- 

 formable with and said to pass gradually into the Laurentian, was, on 

 account of its having limestones associated with it and in conformity 

 with the earlier results of Vennor, placed as an overlying formation. 

 Vennor's work, however, having been continued ten years longer, a{> 

 peared to demonstrate that the Hastings series was continuous with a 

 lower portion of Logan's Laurentian. But Hunt, accepting the earlier 

 work and ignoring the later labors of Vennor, has made out of the 

 Hastings schists and granites the Montalban, and out of the limestones 

 and quartzites the Taconian. 



Again the felsites (orthopyres, petrosilex, etc.), which are known to 

 be the old representatives of the modern rhyolitic lavas, and like them 

 to occur in dikes, or to take the form of lava-flows, ashes, etc., and 

 which were first united with the Huronian, have now been erected into 

 a separate series — the Arvonian. 



Only lithological principles are now used in making these divisions, 

 and every fixct pertaining to the origin and relations of these rocks is 

 ignored ; and since, while it is assumed that all these rocks are sedimen- 

 tary, they were found to occur in dikes and other eruptive forms, it 

 became necessary to hold that all eruptive (including volcanic) rocks 

 were the products of a metamorphic (aqueo-igneous) action. Hence 

 it was claimed that all these rocks had been deeply buried and then 

 denuded,* and most extravagant views became current regarding de- 

 nudation. 



It thus came about that the coarser-grained granitoid and gneissic 

 rocks were set apart as Laurentian ; the gabbros and some of the more 



* Advocated by Lyell in 1833 and later. 



