30 PEOF. T. G. BONJTEY 015" THE 



times appearing as if it stained the mica, sometimes, especially in a 

 more quartzose stratula, like a residuum of an " earthy dust." Now 

 and then I find a granule of quartz, looking very like the indication of 

 an original fragment. A quartz vein occurs in the slide, posterior 

 in date to both foliation and corrugation. I have compared this rock 

 with the most extreme instance of pressure-metamorphism in my 

 collection. Molecular rearrangement is more complete in the former, 

 and the mica flakes in it are about double the size of those in the 

 latter. The Obermittweida rock might be matched with some of 

 the Anglesey schists (e. g. from Holyhead Island) or with exceptional 

 specimens from the uppermost group in the Alps (an average spe- 

 cimen, so far as I know, is more coarsely crystalline). I cannot doubt 

 that it was once a sediment, and that, even if pulverized mica was 

 an important original constituent in this sediment, subsequent 

 molecular changes have seriously modified its structure and given it 

 a crystalline aspect. Were these changes the result of the pressure 

 which has produced the corrugations, and, occasionally, an incipient 

 " strain-slip " cleavage ? To discuss this question would require a 

 lengthy digression, which, after all, would be unsatisfactory, because 

 some of the reasons depend on the results of personal experience, 

 which can hardly be formulated, so I content myself with observing 

 that, while I will not venture to speak positively, I incline to 

 the opinion that the rock had undergone considerable molecular 

 changes before it was corrugated. These modifications, of course, 

 might belong to different stages in a continuous disturbance ; but I 

 am disposed to regard them as separated by a not inconsiderable 

 interval of time. 



My study of this series of specimens from Obermittweida leads 

 me to the following conclusions : — 



(1) That the matrix of the Obermittweida conglomerate has been 

 derived largely from the detritus of a biotite-granite or fairly coarse 

 l^iotite-gneiss, and that a good deal of the felspar has by some means 

 or other been sifted out, so that, chemically, the rock approaches 

 more nearly to the composition of a quartz-mica- (biotite-) schist. 



(2) That the pebbles are derived from a variety of rocks, some of 

 which may have largely contributed to the matrix, but others to no 

 ^eat extent. 



(3) That the structure of some of these included fragments is not 

 that which we generally find in the older part of the Archaean 

 series. 



(4) That the matrix of the conglomerate has undergone a certain 

 amount of metamorphism, so that it may now be regarded as truly 

 crystalline, an amount comparable with that which we often find 

 produced by intrusive masses of granite (I do not, however, con- 

 sider this a case of contact-metamorphism). 



(5) That while the mass has undergone some pressure, to which 

 probably some mineral change is due, I am unable to attribute to it 

 the principal alteration, i. e. this appears to me something more than 

 an ordinary case of " pressure-metamorphism," the matrix exhibiting 



