AUTHORS' ABSTRACTS 21 3 



ments of it are usually seen embedded in the gneiss. Green Keewatin 

 schists are generally turned to hornblende-schist near the contact, but 

 the Couchiching mica-schists are seldom much altered. Dikes of 

 granite or felsite frequently run from the Laurentian gneiss into the 

 Ontarian or Huronian rocks, as they are generally called by Canadian 

 geologists. We have evidently here a section through a pre-Cambrian 

 mountain group, so near its base that some of the meshes run out as 

 unfinished curves, erosion having eaten completely through them. 

 From the steep dip of the schists in the synclines and the width of 

 some of the Laurentian batholites we must infer that these dome- 

 shaped mountains were of considerable height, probably comparable 

 to the highest present ranges. 



Lawson has computed the thickness of the two members of the 

 Huronian (Ontarian) at about five miles each ; so that in some places 

 50,000 feet of sediments, in the upper part mixed with eruptives, 

 must have rested on the old sea floor ; as great a thickness as we find 

 in the sediments preparing the way for later mountain ranges. 



This brought about a rise of the isogeotherms sufficient to produce 

 hydrothermal fusion of the rocks underlying the sediments. 



As the usual theory of mountain building, by lateral thrust, can 

 produce only folds, these domed mountains must have been elevated in 

 some other way. They may be compared with Gilbert's laccolites or 

 I. C. Russell's plutonic plugs, or perhaps more nearly with the struc- 

 ture of the Black Hills, but present important differences from all of 

 these types of mountains. 



The writer suggests that the hydrothermally fused acid Laurentian 

 magma was lighter, both because of its heat and specifically, than the 

 overlying rocks; and so, by the laws of hydrostatics, slowly crept 

 toward the points where the load was smallest, the heavier Huronian 

 rocks sinking toward the lower portions, where they were ultimately 

 nipped in as sharp synclines. 



The region which typically displays this system of Huronian 

 meshes enclosing Laurentian batholites is more than two hundred 

 miles long and a hundred and twenty broad. How much farther 

 similar conditions prevail cannot be known until the Canadian 

 Archaean is more completely mapped than at present. 



The Laurentian has been shown to form eruptive contacts with the 

 Huronian eighty miles north of the Lake of the Woods, and in the 

 Sudbury district, five hundred miles to the east. The Hastings series, 



