204 



PEOF. T. G. BOKNEY ON CKYSTALLINE SCHISTS AND THEIR 



schists, appear to have developed themselves with equal ease in all 

 directions in the enveloping matrix, and any deformations which tbey 

 may present have been subsequently impressed upon them. 



3. The Eatjchwace:e and its Eelation to the Schists, 

 (a) Veil Piora Sections. 



Eauchwacke is a name employed by the Swiss geologists to desig- 

 nate a peculiar rock which occurs, as a glance at their map shows, 

 in a curiously irregular way, generally in elongated patches, over a 

 large part of the Lepontine and Pennine Alps. Commonly it is 

 a rather cavernous or carious-looking limestone of a yellowish, 

 ochreous, or darkish cream-colour, — not unlike some rather soft 

 varieties of tufa — which bruises under the hammer, is rather tough 

 than brittle, and has a dusty surface. It is frequently associated with 

 rather thick beds of anhydrite or gypsum or passes into a dolomite, 

 in which condition it is usually, so far as I have seen, harder and 

 more solid. But, as a rule, the rauchw^acke is a rather soft, non- 

 crystalline rock, and, even when dolomitic, is not so " metamorpbic " 

 in aspect as many of the " dolomites " of the Southern Tyrol. It 

 frequently contains mica, usually more or less silvery. The irre- 

 gular form, tbe variable size, the unequal distribution, and the occa- 

 sional association in flakes of this mineral suggest that it is in great 

 part if not wholly derivative, an idea which is confirmed by the fact 

 that occasionally fragments of mica- and other schists are very 

 abundant*. The Swiss geologists regard the rauchwackef as a Tri- 

 assic rock, and I see no reason for doubting the general correctness 

 of the reference. There can be no doubt that in gome cases it is 

 overlain by Jurassic rocks J. Though once or twice the rauchwacke, 

 from the abundance of mica and the effects of pressure, superficially 

 resembles one of the calc-mica schists, no one, after a study of the 

 rock in the field or even of a fair series of hand specimens, would 

 think of calling it a " metamorphic " rock in the ordinary sense of 

 the word, or of suggesting a close relationship between it and one of 

 the above-described schists. In the one a detrital or fragmental 

 character, when it occurs, is perfectly retained ; in the other, though 

 doubtless the rock had a clastic origin, not a grain of the original con- 

 stituents can be identified. If, then, we are required to regard this 

 soft unaltered rock as tbe basis of the great series of crystalline 

 schists described above, we should be involved in such serious theo- 



* Dr. Grubenmann states that in tbe gypsum be found quartz, pyrite, mica, 

 talc, tourmaHne, distbene, and zircon, mostof wbich are also mentioned by Von 

 Fritscb {lor. cit. pp. 119, 120). He does not, bowever, appear to bave thought 

 it necessary to consider the possibility of their being derivative, but assumes 

 them to be authigenous. I should have thought that the appearance of the rock 

 vv^ould have suggested the need of rigorous demonstration. 



t For brevity I use tlie term inclusively, the pi'esence of dolomite, anhydrite, 

 or g5^psum being, for my present purpose, of subordinate importance. 



X Von Fritsch {loc. cit. p. 3) says the Lower-Lias Belemnites Oppeli has been 

 found on the Nufenen Pass. 



