MACKIE — RTPPLE-DRTFT IN MICA-SCIIIST. 



203 



fully following all the transitions from simple to complicated that 

 the true nature of some of the contorted bands of ripple-drift can be 

 understood. In no other way can we explain the various curious 

 structures that are met with in some localities, as, for instance, the 

 neighbourhood of Arroquhar, whence I have a number of speci- 

 mens prepared so as to show the structure in a very striking manner. 

 The value of the fact of the occurrence of the ripple-drift consists in its 

 proving so very conclusively that the schists were originally deposited 

 in the same manner as ordinary sedimentary rocks, under the in- 

 fluence of gentle currents. Moreover, in studying the microscopical 

 structure of various mica-schists, one may see some of the original 

 grains of sand, although since somewhat altered, and may clearly per- 

 ceive that the present characters of the rock have been produced by 

 a general crystallization of the various constituents, so as to give rise 

 to what is known as a metamorphic rock. In one thin microscopical 

 section proofs were exhibited of all the leading facts of metamorphism. 

 We had the evidence of deposition from a gentle current of water, of 

 subsequent compression, and of the alteration of the whole into a 

 highly crystalline rock." 



The specimen of ripple-drift in mica-schist which we figure in 

 Plate XI. is from between Arroquhar and Tarbet, and our engraving is 

 from a drawing which Mr. Sorby has kindly sent us. "It was," he says, 

 "somewhat difficult to convey the true character of the rock, but will 

 be more easy on a wood-block, for it is really a white pattern on a 

 black ground. You must bear in mind that the small black stratula 

 pass completely into the larger bands. I mean the, stratula iu the line 



Fig. 1. — Stratula passing into larger bands. 



a, b, c are continuous with a a,b b, c c. (See Fig. 1.) Also, these dark 

 stratula become thicker near the junction, or, as is really the ease, the 

 white bands of quartz, which were originally sandy, thin out at the 

 ends. Of course you understand that, in the case drawn, the whole 

 has been much disturbed and greatly contorted, the only part at all 

 in its original state being the right-hand Bide of the central band, 

 where 1 have drawn a hand pointing." (See Plate XI.) 



