[ells] CANADIAN GEOLOGICAL NOMENCLATURE 2S 



disposition of the intervening gneiss, limestone and scliist series. This 

 change, from the middle position as regards the divisions of the Lauren- 

 tian, was not possible till the establishment of the fact some time later, 

 that the anorthosites were an intrusive group of rocks, and entirely 

 distinct from the limestones which they traversed. This did not take 

 place till after the investigations of Yennor in the area north of St. 

 Jerome in 1877, the history of which may be briefly summarized. 



The controversies that have been waged with more or less per- 

 sistence, concerning the relative positions of the several members of the 

 crj^stalline rocks appear to have been largely brought about by the at- 

 tempt to reconcile widely diverging views with a hard and fast scheme 

 of classification, which had been devised in the early days of their study 

 in the field. Thus it was apparently taken for granted that the greater 

 part of the Laurentian rocks* were originally of sedimentary origin. 

 This opinion was doubtless based on the presence of a bedded, banded 

 or foliated structure visible in the greater portion of the old granite and 

 gneiss formations. It was, however, recognized by Logan, at an early 

 stage of the investigations, that there was a vast underlying mass of 

 granite-gneiss, which was older than the grayish and often garneti- 

 ferous gneiss, limestone and quartzite series. What appears to have been 

 the chief misleading influence in the discussion was the supposition that 

 the anorthosites, which also in places presented a foliated structure, 

 represented altered sediments. 



In the supplementary chapter on the Laurentian, given in the Geo- 

 logy of Canada, 1863, the relation of the anorthosites to the limestones 

 and associated gneiss of the Grenville series is considered. The in- 

 terruption of the limestone bands of Morin and of the adjacent town- 

 ships, is clearly pointed out, but the eruptive nature of the contacts, at 

 this place, was not at that time distinguished. It was assumed from the 

 relations of the two series, as there seen, that there was a strong 

 probability that the " anorthosite rock overlies the whole Grenville series 

 unconformably, and that the mass of it on the west side of DeSalabcrry 

 is an outlying portion." Further it is there stated that " if, on ex- 

 ploration to the eastward of the Trembling Mountains, it should be 

 further ascertained that the two inferior bands of the Grenville series 

 disappear on reaching the margin of the anorthosite, it may be con- 

 sidered conclusive evidence of the existence in the Laurentian system of 

 two immense sedimentary formations, the one superimposed unconform- 

 ably on the other, with probably a great difl'erence in time between 

 them." 



From the laying down of this principle then, as to the structure of 

 the several divisions of the Laurentian system, much of the subsequent 



