THEIB CONTACTS. 315 



glomerates have all been altered more or less by the secondary enlarge- 

 ment of the component grains and the deposition of interstitial silica, 

 and one can frequently obtain specimens showing a perfect gradation 

 from the loose, friable sandstone to the almost completely vitrified 

 quartzite, there being every possible phase between these two extremes. 

 In the vicinity of the gneiss and granite both the quartzite and gray- 

 waeke are altered into mica schists. 



THE LAURENTIAN ROCKS. 



The Laurentian to the north of hake Huron is chiefly represented by 

 gneiss which differs from granite only in being foliated. Frequently 

 this foliation is quite distinct, though sometimes it is obscure and occa- 

 sionally it cannot be detected at all, the rock being then indistinguishable 

 from ordinary irruptive granite. Intimately associated with the gneiss 

 are certain true granites and syenites, non-foliated and varying in tex- 

 ture from coarse to fine-grained. Although in many cases there is the 

 clearest evidence that these granites cut the gneiss, yet for the following- 

 reasons both may be considered to have had a common genesis : 



1. The presence of streaks and lenticular patches of darker-colored 

 material in the midst of these non-foliated areas, all of which have a 

 more or less constant direction. 



■2. The frequent absence of any sharp line of division I letween the gneiss 

 and granite, the one passing into the other by insensible gradations. 



3. The occurrence of dikes and veins of pegmatite cutting both the 

 gneiss and one another clearly belonging to the same period, although 

 the fact of one cutting the other would seem to indicate a lapse of time, 

 which in this case was doubtless of small import. 



4. Their close resemblance in composition, appearance and behavior. 

 These masses of granite may therefore be regarded as non-foliated 



areas of gneiss, representing simply certain irruptions from the same fluid 

 magma from winch the gneiss itself has solidified, and although a suffi- 

 cient time has elapsed to allow of the more or less complete consolida- 

 tion of the gneiss, yet they represent the same age in geologic time. 

 The first-formed crust was necessarily thin and weak, so it is not surpris- 

 ing that the basement complex exhibits such frequent evidence of the 

 upwellings from beneath of the fluid magma. The non-foliated character 

 of many of these masses may have been due to their very gradual intru- 

 sion and the absence of pressure during tin,' process. 



Gexeral Cotjrse of the Contact ox Lake Huron. 



On Lake Huron the contact between the Laurentian and Huronian 

 comes out on Killarney bay, a mile and a half north of Killarney post 



