[ELIS] CANADIAN GEOLOGICAL NOMENCLATURE 27 



rence counties in the United States, and Eawdon, Grenville and Hasting? 

 in Canada, are parts of one great series, and at present I see no evidence 

 for excluding from this series the associated ISTorian recks. Whether this 

 series as a whole will eventually retain the name Upper Laurentian or 

 whether it will he found more convenient to designate it the Huronian 

 sj'stem, does not much signify." 



Eeferring again to the question of the origin of the Labradorite 

 rocks in 1877-78 Dr. Selwyn remarks, " if as is admitted— which in view 

 of the usual associations of Labrador felspars, is the most probable sup- 

 position, — that these anorthosite rocks represent the volcanic and intru- 

 sive rocks of the Laurentian period, then also their often massive and 

 irregular, and sometimes bedded character, and their occasionally inter- 

 rupting and cutting off of the limestone bands as described by Sir W. E. 

 Logan, is readily understood by 'any one who has studied the strati- 

 graphical relations of contemporaneous volcanic and sedimentary strata 

 of Palaeozoic, Mesozoic, Tertiary and recent periods. Chemical and 

 microscopical investigations both seem to point very closely to this as 

 the true explanation of their origin/' 



As the study of these rocks proceeded, further light on their re- 

 lations was obtained. In several cases the contacts between the anor- 

 thosites and the limestones wore found to be similar to those observed 

 in the case of true dioritic masses. Vennor pointed out several cases of 

 such contacts in his summary report for 1877 in the country north of St. 

 Jerome, and these were regarded at the time as so conclusive, that Dr. 

 Selwyn upon consideration of these points was forced to state that " if 

 the foregoing determinations of Mr. Vennor were correct, they seem 

 very conclusively to prove, what I have already stated to be my opinion, 

 viz., that the Labradorite or Norian rocks of Hunt do not constitute an 

 unconformable upper Laurentian formation, but occur in part as unstra- 

 tifled intrusive masses and in part as interstratifications with the ortho- 

 clase gneisses, quai-tzites and limestones of the Laurentian system as 

 developed in the Grenville region and mapped by Sir W. E. Logan.'' 



Concerning the later investigations on this series of rocks it may be 

 said that the recent work of Adams on the origin and relations of tlie 

 anorthosite masses, previously supposed to represent the tipper member 

 of the Laurentian system, has very conclusively proved their igneous 

 character, and that they are true intrusive rocks in the limestone and 

 gneiss of the Grenville series, and of later date than these sediments. 

 The establishment of this point in the structure of these old portions of 

 the Laurentian of Canada, has had a marked bearing on the solving of 

 other questions relating to the divisions of the Laurentian proper and 

 to the overlying rocks of the Hastings and Grenville series. 



