1889.] 



Contact Metamorphism in Silurian Rocks. 



203 



of us in the case of the Obermittweida conglomerate,* the mica 

 appears to be partly original, partly secondary ; indeed, repetition 

 may be saved by referring to the published description of the grit 

 from the inner part of the fold at that place, merely stating that 

 in it secondary changes appear to be rather more pronounced than 

 in the present specimen. 



The other specimen is a homogeneous, compact, very fissile rock, 

 like a schistose slate or phyllite. Microscopic examination shows that 

 it consists of a micaceous mineral and quartz, with occasional granules 

 of epidote (rare), of an impure kaolinitic mineral, and flakes or perhaps 

 rods of iron oxide. In most parts of the slide these are the chief 

 constituents. The flakes of the first-named mineral occasionally, in the 

 more quartzose parts, are about 0'0025" long, but generally less. It 

 varies from almost colourless to a pale olive tint, but sometimes is 

 slightly brown. It often resembles a mineral common in some of the 

 oldest green slates, and some of it is probably a chlorite. The associated 

 quartz is generally very minute, but rather larger grains of distinctly 

 fragmental aspects, up to about 0"004" in diameter, occur, especially 

 in certain layers, and there are others consisting mainly of quartz. 

 It is, however, possible that some of these may be veins, indeed, one 

 of the most quartzose certainly is, but others are hardly less certainly 

 true stratulaB, indicative of former bedding. The sharp flexures in 

 these, the "rucking" of the layers of mica, and the development in 

 parts of the slide of an incipient strain-slip cleavage (ausweichungs- 

 divage) indicate that the rock, subsequent to partial mineralisation, 

 underwent great pressure. In short, macroscopically and microsco- 

 pically, it bears considerable resemblance to those " satiny " slates or 

 phyllites, the alteration of which is, to a very large extent, due to 

 pressure. At the same time, when we examine the individual plates 

 of mica and the structure of the folds with a high power, we are led 

 to think that some subsequent modifications have taken place. As 

 has been described in the paper already mentioned, each individual 

 flake of mica appears as if the process of formation had been 

 completed in situ, or that in some way or other it had been enabled to 

 f right itself 7 ' after the distortion which is usually produced by these 

 great earth-movements. So also the layers of mica, as it were, slightly 

 bristle with the projecting ends of mica flakes ; at the sharpest part 

 of a fold they have lost the " strained " look, like a rope beginning to 

 part, and the incipient plane of strain-slip cleavage is often "soldered 

 up." The changes have taken place (though to a much less extent), 

 which are described in the paper by one of us on the effect of con- 

 tact metamorphism on phyllites at Morlaix.f Probably we shall not 

 be wrong in ascribing them to the elevation of temperature, connected 



* ' Quart. Journ. Geol. Soc.,' vol. 44, p. 29. 

 t k Quart. Journ. Geol. Soc.,' vol. 44, p. 12. 



