MACKIE — EIPPLE-DEirT IX MICA-SCUIST. 
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- 
meiis 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 Tarbct, and our engraving is 
from a drawing which Mr. Sorby has kindly sent us. "It was," be 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. Ton must bear in mind tliat the small black stratula 
pass completely into the larger bands. I mean the, stratula in the line 
Fig. 1. — Stratula passing into larger bands. 
a, h, c are continuous with a a,hh,c c. (See Fig. 1.) Also, these dark 
stratula become thicker near the junction, or, as is really the case, 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 side of the central band, 
.where I have drawn a hand pointing." (See Plate XL) 
