1863.] 



SOKBY MICA-SCITIST. 



405 



Fig. 2. — Contorted "Ripple-drift" in Mica-schist from Muchuls, 

 exhibiting " Cleavage-foliation." (Magnified 3 diameters.) 



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The micaceous parts possess a well-marked cleavage-foliation : 

 the crystals of black mica do not lie in the line of the stratula (b c), 

 nor in that of the compound bands (a b, c d), but have one prevailing 

 direction, as shown in the drawing. Judging from the small con- 

 tortions, and from the alteration of the inclination of the stratula to 

 the compound bands, which originally cannot have been more than 

 30°, but is now often as much as 90°, this direction is perpendicular 

 to that of the pressure which produced the disturbance, and, no 

 doubt, developed slaty cleavage in the rock ; so that, when subse- 

 quently metamorphosed, the crystals of mica were formed with their 

 flat surfaces and cleavage in the plane of the slaty cleavage. In mica- 

 schist which has been contorted since it was metamorphosed and 

 possesses only stratification-foliation, the crystals of mica lie with 

 their flat surfaces in the plane of deposition of each part of the 

 ripple-drift, so that it might be argued that they had been mechani- 

 cally deposited in that position. In the case of the specimen I am 

 now describing this explanation is, however, quite inadmissible ; for 

 the crystals of mica could not possibly have derived their present 

 arrangement from deposition and subsequent mere mechanical dis- 

 turbance, and must, I contend, have been formed in the midst of the 

 rock by chemical and crystalline action. Hence this one thin 

 section does by itself afford most complete proof of original deposi- 

 tion as a stratified rock, of subsequent compression and the develop- 

 ment of slaty cleavage, and of the final alteration into a crystalline 

 schist. As throwing light on this last change, I may here state that 

 I have recently prepared a thin section of mica-schist from the same 

 locality as that just described, which, on the whole, is extremely 

 altered and crystalline. Some parts are solid, clear quartz, and 

 others a mixture of black, green, and white mida and of pink 

 garnets; but mixed up with these highly crystalline portions are 

 others of much finer grain, which, though not having the structure 



