18 PEOr. T. G. BOirSEY ON SOME EESTJLTS OF 



eacli material there were stratulse of the other ; pressure, apparently 

 acting first roughly at right angles to this banded structure, caused 

 certain mineral changes, and gave rise to an incipient metamorphism, 

 even an " embryonic " foliation. Further pressure crumpled up the 

 beds, producing corrugation of the mineral bands, and in some cases 

 " Ausweichungsclivage," possibly also continuing a little further the 

 mineral change. Still the detrital origin of the rock remains 

 indubitable, and (what is very important) the original stratification 

 can be clearly recognized, and no new mineral-banding is introduced 

 as the result of pressure. Then the temperature of the mass is 

 raised by the intrusion of igneous rock ; much more considerable 

 mineral changes take place, the minerals already existing are 

 enlarged, new minerals are formed, the old divisional planes are 

 soldered together, the whole mass is consolidated and rendered 

 crystaUine *. Still a perfectly normal schist has not been produced. 

 "Why ? I think we may express the chief reason in a homely way by 

 saying that N'ature has baked or stewed her pudding in too " quick " 

 an oven. The element of time is wanting ; the process of segre- 

 gation and development, though for a while intense, has been too 

 soon arrested; a longer time at a somewhat lower temperature 

 would, I believe, have allowed of a more complete separation. 



It is remarkable that the process of contact-metamorphism seems 

 very unfavourable to the formation of a felspar. The analyses 

 which have been made and are often quoted indicate that though 

 the ordinary muddy sediments are commonly not rich in potash or 

 soda, these substances are frequently present in sufficient amount 

 to form some felspar ; but in these rocks we get quartz, mica, and 

 an alumina-subsilicate (chiastolite or andalusile), we may perhaps 

 get garnets, but not felspar. I have, indeed, read of its occurrence, 

 but though I have examined a good manj'' specimens, I never saw 

 an indubitable felspar, and I know that some of the instances which 

 have been quoted are more than dubious. 



But, whatever be the explanation of this peculiarity, the cases 

 which I have described seem in my opinion to justify the following 

 inferences ; that the foliation in certain mica-schists is a record of 

 an original stratification of the materials; that these rocks were 

 once composed of stratulae of sand, silt, and mud ; that the foliation 

 is a record of the original bedding, and that the latter, though it 

 may be affected and sometimes even obliterated by subsequent 

 mechanical, followed by chemical, alterations, can in many cases be 

 readily recognized, and can be distinguished from the cleavage- 

 foliation and pseudostromatismf caused by these changes. Further, 

 the inference seems reasonable that these normal schists, early 

 in their history, have passed through an environment different 

 in some important respects from that of indubitably Palaeozoic 

 rocks — different, at any rate, in this, that whole regions instead of 

 very restricted areas were similarly and simultaneously affected. 



* Compare thew to microscopic sections on Plate IT. 



t For definition of this term, see my Address, Quart. Journ, GFeol. Soc, 

 Tol. xhi. Proc. p. 65. 



