IN THE NEIGHBOURHOOD OF StTDBTJET, CANADA. 43 



structural and mineral changes much depends upon the constituents 

 of a rock. Under certain circumstances, such minerals as epidote, 

 chlorite, viridite, hydrous white mica, quartz, &c., readily form ; a 

 clean sandstone, for instance, is readily cemented into a quartzite, 

 the calcareous constituents of a rock will crystallize, while the argil- 

 laceous are absolutely unchanged. But making all allowance for 

 this, and confining my attention to the finer portion of the detrital 

 materials, I find them in the case of the latter group more altered 

 (especially where brown mica is developed) than I have ever yet seen 

 in any Palaeozoic fragmeutal rock, even the oldest, or even in the 

 typical " Pebidian " of this country, except when affected by contact- 

 metamorphism. Hence, I think, our choice lies between the second 

 and third hypotheses, and, as at present advised, I incline to the 

 latter ; viz. that two distinct groups, of which, at any rate, one is 

 Pre-Cambrian, are included under the name Huroniau. 



(4) It is a curious coincidence that fragments of lava bearing a 

 general resemblance to those which in Great Britain are found, cer- 

 tainly or presumably, rather below the level which appears to be 

 the natural base of the Cambrian series (i. e. in the so-called Pebi- 

 dian), should occur so abundantly in a formation which also seems 

 to be the record of a late phase in Archaean history — one of the 

 concluding chapters of the volume. 



(5) It seems also worth note that many of the fragments have 

 assumed structures characteristic of Laurentian rocks, such as the 

 peculiar " intermediate " structure, neither normal granite nor nor- 

 mal gneiss, — or a somewhat foliated structure, resulting from mecha- 

 nical, followed by chemical change, — prior to the formation of 

 these conglomerates. Also that certain other important changes, 

 apparently of a somewhat segregatory nature, had taken place 

 in other rocks, possibly of igneous origin. The occurrence, too, of 

 fragments of a true schist, similar to those met with in old conglo- 

 merates elsewhere (e. g. Charlton Hill, Salop) and not at all unlike 

 one which I obtained in situ near Straight Lake, is also interesting 

 as showing that ordinary schists and gneisses, of comparatively fine- 

 grained structure, existed at that period as well as granitoid rocks. 



(6) The changes which have taken place in the more altered 

 " Huronians " show that a gneiss might conceivably result from the 

 alteration of a felspathic greywacke or a mica-schist from a muddy 

 sandstone, so that it is possible for a series of banded gneisses and 

 schists, of moderate coarseness, to have been produced by the meta- 

 morphism of a sedimentary series. Other specimens, however, indi- 

 cate the possibility of certain gneisses and certain schists being due 

 to the metamorphism of rocks originally igneous. 



(7) While in the case of some of the f ragmen tal rocks there is 

 reason to believe that the mineral changes in the included fragments 

 occurred prior to their being detached ; others (which seem origi- 

 nally to have been very compact, possibly porphyritic lavas) appear 

 to have undergone a change together with the matrix in which they 

 are imbedded. It is possible that these breccias may be of volcanic 

 origin, at any rate it appears probable that from " trachytic " mate- 



