204 W. H. COLLINS 



Their mountainous aspect is enhanced by the fact that they con- 

 sist of white, Lorrain quartzite and, when seen from any consider- 

 able distance, look as if they were covered with snow. The last 

 of the five cross-sections in Figure 2 is through Lacloche Mountains. 

 The sediments forming these mountains are Precambrian (Huro- 

 nian), and the intrusive granitic bathoHths are late Precambrian 

 (Killamey) . . 



Figure 2 has been introduced for the purpose of demonstrating 

 that the associated phenomena of regionally deformed strata and 

 batholithic intrusion are results and criteria of mountain-building 

 and that by sufficiently protracted erosion a mountain system may 

 be reduced to a peneplain having the topographic appearance and 

 geological characteristics of northern Ontario. Endeavor will now 

 be made to apply these criteria to the Precambrian Shield and to 

 outline the chief events in the physiographic history of the north- 

 eastern part of Ontario, proceeding from the present as far back in 

 time as the geological record has been interpreted. 



Figure 3 shows the distribution of the essential geological ele- 

 ments in northeastern Ontario with which this thesis is immediately 

 concerned. It also contains a tabular record of the geological 

 periods. Periods not represented by rock formations in the region 

 are shown in brackets. The larger spaces represent gaps in the 

 record of intervals of erosion, and an attempt has been made to 

 indicate, conservatively, by the size of the space, the length of 

 time unrepresented by rock formations. 



Let us take that part of the record from the Ordovician period 

 to the present. Only a few of the series of formations are present — 

 the Ordovician, Silurian, part of the Devonian, a httle of what Mr. 

 Keele regards as Cretaceous,' and the Pleistocene. All these forma- 

 tions He horizontally or dip at very low angles. The Ordovician, 

 Silurian, and Devonian are limestones, shales, and sandstones that 

 contain fossil remains of marine animals. The Cretaceous contains 

 beds of Hgnite and was presumably laid down on land or very near 

 land. The Pleistocene consists of glacial bowlder clay and strati- 

 fied clays and sands laid down in postglacial lakes. The careful 

 studies of glaciaHsts, Coleman, Johnston, Taylor, and others, have 



' Summary Report, Geol. Surv., Can. (1919), Part G. 



