44 PEOP. T. Gr. BOXNET Om THE h:d'eo:nta]S' seeies 



rials quartz-mica rock has been developed, -which, however, differs 

 (chiefly in the absence of foliation) from a normal mica-schist. 



(8) The evidence of these Huronian rocks, so far as it is positive, 

 is in favour of the theory which regards the more coarsely crystal- 

 line gneisses and schists as produced, as a rule, only in early Archaean 

 times ; and so far as it is negative, it is against the theory which 

 regards them as metamorphosed sediments of Palaeozoic or even later 

 age, because we find in them an approach, though an incomplete one, 

 to the structure of crystalline gneisses and schists, and this approach 

 is nearer than I have yet found in any rock indubitably Palaeozoic*. 



Postscript. — A common thread of thought and purpose runs 

 through these three papers; they form a kind of trilogy; may I, 

 then, be allowed to point the moral in a brief epilogue ? 



The first illustrates the effects of pressure on Palaeozoic sediments ; 

 the result is micro-miner alogical change only, the production of tiny 

 films of mica, of specks of secondary quartz, an enlargement probably 

 of clastic granules of the same. Thus a microf oliation only is pro- 

 duced, which, strange to say, appears to be parallel to the original 

 stratification andindependent of the pressure w^hich has subsequently 

 cleaved the rock. If the temperature of a mass, thus modified, be 

 considerably raised by the intrusion of igneous rock, further and 

 more active chemical changes occur, the minerals already microsco- 

 pically present grow larger, whUe others are produced. The result 

 is a moderately good imitation of one of the fine-grained mica- 

 schists. 



The next paper illustrates the changes in a fragmental rock of 

 unknown geological age, which has certainly been compressed, which 

 may have had its temperature raised, though not by intrusive igneous 

 rocks, and in which considerable change has been produced ; the 

 result, however, in this case also does not quite accord with a 

 typical fine-grained mica-schist, though in one instance it comes 

 very near to it. 



In the last case we are dealing with rocks, certainly of great an- 

 tiquity, which I suppose all would admit to be, in the main, Pre-Cam- 

 brian. Here we find changes very similar to those last described ; 

 these also have not produced typical gneisses and crystalline schists, 

 and they further distinctly testify that when they were formed, such 

 rocks already existed, and mineral changes occurred, seemingly with 

 more facility than in later days. To what conclusions these results 

 point, it is needless to suggest to a careful reader. 



* This paper was completed (except for two or three triyial details) last 

 August. Since my return to London in October, I have had the opportunity 

 of reading the most valuable paper hj Prof. E. S. Irving, " Is there a Huro- 

 nian Formation ? " (Amer. Journ. Sci. vol. xxxiv. pp. 204, 249, and 365). 

 We appear to have arrived independently at verv nearly the same conclusion. 

 I think, however, that the Sudbury rocks exhibit, as a group, rather more alter- 

 ation than those from the vicinity of Lake Huron, as described by him, and as 

 confirmed by the specimens sent to me by Dr. Selwyn. Possibly^he Sudbury 

 rocks may be a shghtly older group, the equivalents of his " iron-bearing (Aui- 

 mike) series," p. 216. 



