EELATION TO MESOZOIC ROCKS IN THE LEPONTINE ALPS. ^^6 



far as my experience goes, they are rare and very local ; and these 

 arise from one of two causes, viz. : sometimes the base of a sedi- 

 mentary group is a kind of " arkose " — i. e., is composed of frag- 

 ments, rather uniform in size, of the underlying crystalline rocks. 

 Such material, after pressure and the consequent development of 

 minute filmy mica, is difficult to distinguish, especially when im- 

 perfectly exposed, from certain members of the crystalline series 

 when they have been much crushed. h?ometimes also a dark 

 micaceous schist, when crushed, is locally difficult to distinguish 

 from a dark argillaceous sediment in which severe pressure has 

 developed a secondary mica. In short, so far as my observations 

 go, the direct effect of pressure in promoting crystallization is not 

 very great *, and the minerals which may be connected with it are 

 usually minute. It appears to be most efficacious when acting 

 upon a rock already crystalhue, and especially in producing changes 

 more or less of a paramorphic nature. But its result, even in the 

 case of such a rock, is usually to diminish the size of the crystalline 

 constituents, to convert coarse- into fine-grained rocks, and thus it 

 causes a rock to recede from rather than advance towards what may 

 be called a maximum stage of alteration. 



In short, the result of my investigations differs widely from the 

 conclusions announced to the Geological Congress and embodied in 

 the following quotation f : — " That by the plication of the Alps the 

 constitution of the rocks has been completely changed is most dis- 

 tinctly proved by an examination of the sedimentary rocks ; because 

 the latter can be studied in an unaltered condition in adjacent 

 localities. The commonest changes met with in connection v/ith 

 folding are .... marmarosis of the limestones .... Develop- 

 ment of new minerals (garnet, staurolite, mica) in places that have 

 undergone crushing .... Even Liassic slates with fossils have 

 been converted into garnetiferous mica-schists, &c. The boundary 

 between the old crystalline schists and real sediments in the Alps 

 has by such processes of dynamic metamorphism been obliterated, 

 and the proper character of the rock is altered so as to render 

 recognition impossible. When we see in true sediments new 

 minerals developed by the progress of the mechanical metamorphism 

 (magnetite in the crushed oolitic ironstojjc of the Windgiille, garnet 

 in the Belemnite-slates of Scopi), the question arises for the crys- 

 talline schists of this and the neighbouring regions : AYhich 

 minerals are original, and which have been produced subsequently 

 by orogenetic processes ? '' It is, then, obvious that I dispute tlie 

 accuracy of most of the statements on whicli this question is 

 founded, and in so doing have endeavoured to show that it can be 

 answered without the aid of a geological Qjldipus. 



* See, for a fuller discussion on this point, my note in the GeoL Mag. 18S9, 

 dec, iii. vol. vi. p. 48J}. It must be remembered also (a point whicli seems to be 

 often overlooked) that the formation of new minerals takes place during the 

 diminution, not during the increase of pressure. This matter cannot be 

 discussed here, but I am preparing some notes on it for publication. 



t ' Nature,' Sept. 27, 1888, p. r)24. Passages are omitted to which 1 take 

 no exception. 



