Io8 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



cannot now be regarded as conclusive ; and even the opinion of a 

 skilled microscopist may be of little value unless he has served a 

 long apprenticeship to this particular kind of work. Its problems 

 are far more difficult, far more complicated, far more misleading 

 than those which present themselves in the study of the igneous or 

 of the ordinary sedimentary rocks. On this I may venture to insist, 

 as I can speak from personal experience. Pive years, at least, of 

 fairly hard work have shown me how many of my earlier im- 

 pressions were incorrect, and how much I have yet to learn in 

 regard to this vast, but strangely fascinating, subject ; but they have 

 also given me some confidence in regard to certain general results, 

 which entitles me to demand a rigid proof of any assertion which 

 makes against them. I will therefore briefly recapitulate the evi- 

 dence as to the age of the crystalline rocks in the districts which 

 I have already described. In the Alps the scJiistes lustrees* (or 

 uppermost group of schists) have by some geologists been claimed 

 even as altered Trias. As the above name appears to me to have 

 been used rather inclusively, it is possible that some portions of 

 the group may be representatives of Palaeozoic or Mesozoic strata. 

 In such case, however, it will be found that they are but slightly 

 altered, and so have no right to be included in it. Confining 

 myself to the true schists, such as have been described in a former 

 part of this address, I can only say that they must be much older 

 than any rock to which we can certainly assign a date. The 

 Jurassic and Triassic strata of the Alps are, at most, but slightly 

 altered, even where they have been most tightly nipped between 

 huge pincers of crystalline rock. It is a misuse of the term, as I 

 have defined it, to call them schists. Sericite and other secondary 

 minerals have undoubtedly been developed ; but these (where of any 

 significance) are on the most minute scale, and one of these " phyl- 

 lites " has hardly more relation to one of the crystalline mica-schists 

 than a Hanoverian medal has to a genuine sovereign. In the less 

 disturbed districts the Secondary strata rest upon the edges of the 

 crystalline schists, and both are in their normal condition. Base- 

 ment conglomerates and breccias in the former contain fragments of 

 the difi'erent types of crystalline rocks, from the coarser gneisses to 

 some at least of the later schists. In like manner the Carboniferous 

 rocks of the valley of the Hhone exhibit practically no real approach 

 to the crystalline schists. To assert that " at Vernayaz, near 

 Martigny, the Carboniferous strata can hardly be separated from the 

 schists," is only thus far true, that as in places the base of the 

 Carboniferous series is a conglomerate or arkose, composed of de- 

 tritus from the gneiss, and both have undergone great compression, 

 and have been somewhat decomposed, a spot can now and then be 

 found where it is difficult to draw the line with absolute precision ; 

 but, as a rule, although some secondary minerals, generally minute 

 in size, have, under favourable circumstances, been developed in the 

 Carboniferous series, the two groups are perfectly distinct. This 



* These appear to be identical with the "Biindner Schiefer" of Q-erman- 

 speaking geologists, those which I have called " Brown-bedded " schists. 



