330 A. E. BARLOW — LAURENTIAN AND HURONIAN ROCKS. 



ceous quartzites near the line of contact, which frequently resemble in 

 character and composition the more evenly laminated gneisses, has been 

 referred to as evidence of such a transition ; but even in such a case the 

 bedded character of the Huronian is in strongly marked contrast to the 

 granitic aspect of the gneiss. 



Conclusions and Facts supporting Them. 



In conclusion, then, the following facts seem to prove beyond a doubt 

 the irruptive nature of this Laurentian gneiss and its magmatic condition 

 at a time subsequent to the petrifaction of the Huronian sediments : 



1. The diverse stratigraphic relations of the two rocks along their 

 line of junction. Most frequently the Huronian strata dip into or under 

 the gneiss, although often this position is reversed and the Huronian 

 beds are seen superimposed on the gneiss with perfect conformity. In 

 many instances the two rocks occupy vertical positions side by side and 

 occasionally the gneiss has been seen dipping away from vertical Huro- 

 nian strata. Huronian rocks have also been seen resting unconformably 

 on the upturned edges of Laurentian gneiss. Sometimes, where the sin- 

 uosities of the line of outcrop of the gueiss were too abrupt to be followed 

 by the stratified Huronian. the latter rocks have abutted on the gneiss 

 as on an irruptive mass. These different phenomena can all be readily 

 and naturally explained by the irruption of the gneiss, while on the 

 hypothesis of an aqueous origin such explanation must be difficult and 

 unsatisfactory. 



2. The alteration of the sedimentary rocks along the line of junction 

 is a feature that has been invariably noticed where the contact has been 

 examined. 



3. The inclusion of angular fragments in the mass of the gneiss which 

 are clearly referable to the adjacent sedimentary strata. Near the line 

 of junction these detached pieces have a clear and sharp outline, while 

 further in the mass, where the}' have undergone partial fusion and ab- 

 sorption, their outlines are blurred and indistinct. 



4. The occurrence of gneissic intrusions as well as more coarsely crys- 

 talline apophyses of pegmatite both interlaminated with and transverse 

 to the bedding of the Huronian rocks. These intrusions are distinctly 

 irruptive and can often be directly traced to their source in the larger 

 area of gneiss in the vicinity. 



5 The absence of limestones, slates or quartzites or, in fact, any species 

 of rock indicative of ordinary sedimentation for the quartzites and mica 

 schists sometimes seen interlaminated with the gneiss are simply 

 quartzose and micaceous phases of the more common feldspathic gneiss. 



