STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 379 



sidered as of marine or of terrestrial origin. They are separated by 

 a fairly sharp boundary from the known marine portion of the 

 Grenville to the south, in which much limestone is found. This 

 boundary represents approximately the position of the ancient 

 shore line, if the sediments to the north are of terrestrial origin; 

 if the sediments to the north are marine, it represents a topographic 

 break, separating an area of very shoal water deposition to the 

 north from an area of deeper water deposition to the south. In 

 the latter case, the sediments of the Grenville must have been 

 derived from a land area occupying the site of Hudson Bay. 



At the end of the Grenville period of deposition, uplift appears 

 to have taken place, and a period of erosion ensued. This was 

 sufficiently long, to remove all or the greater part of Jhe Grenville 

 series from much of the interior plateau, where presumably it was 

 thinnest, since in the area between Lak^) Mistassini and James Bay 

 the supposed Grenville has been found in only two places between 

 the lavas and the Mattagami sediments. 



The emergence just described appears to have been accompanied 

 or closely followed by intrusions of granite in the southern portions 

 of the plateau. This "older granite" has not yet been recognized 

 farther north than Lake St. John. There is some evidence to 

 show that folding movements also accompanied these granite intru- 

 sions, causing deformation of the rocks of the southern edge of the 

 plateau. After the intrusion of the granite and the folding move- 

 ments, if any, were ended, masses of anorthosite and feldspathic 

 gabbro were intruded. These have been found as far north as Lake 

 Chibougamau, but are more numerous and larger in the southern 

 districts, where the earUer granite occurs. The localization of two 

 such widespread intrusions within similar limits is probably due to 

 some common cause; perhaps to the presence of lines of weakness 

 in the older rocks caused by the folding just mentioned. No 

 important orogenic movement appears to have accompanied the 

 anorthosite intrusion. 



The next event in the history of the region was the deposition of 

 the Mattagami series, which may or may not have been accom- 

 panied by marine submergence. The thickness of the basal con- 

 glomerate of the series indicates that at least this member probably 



