﻿Vol. 64.] QTJANTITATITB METHODS TO THE STUDY OF ROCKS. 197 



have been overlooked, if the slates had uot been wetted by a shower 

 of rain ; but, in order to see them to perfection, the surface of the 

 slate should be ground flat and smooth, and coated over with a thin 

 layer of so-called ' negative ' varnish. 



In my specimens the plane of cleavage on which the structures 

 are seen is inclined at about 45° to the stratification, and in calcu- 

 lating out the results allowance has been made for this, and also 

 for the change of dimensions when the cleavage was developed. The 

 existence of this constitutes, however, a great advantage, since the 

 rock has been compressed so as to be very nearly solid, the cavities 

 amounting to only 0*5 per cent. ; whereas in analogous rocks 

 from the Coal-Measures they amount to over 13 per cent. More- 

 over, the character of the rock is eminently favouralDle for exhibiting 

 the structure, since the fine-grained material is a pale green, and 

 the coarser a dark green. The microscopical structure shows clearly 

 that practically the whole material is a volcanic ash ; and the 

 structure in many cases is as though this had been deposited from 

 above, with little or no drifting along the bottom or sorting by a 

 current. In the following account of my specimens, I adopt for 

 calculation, etc. the general conclusions already explained. PL XIV 

 is a reproduction of a photograph of a case where the current was 

 so gentle that only very fine-grained green material was deposited in 

 just the same creamy semi-liquid condition as recently-deposited 

 clay in which the amount of included water is about 80 per cent., 

 so that it can be easily washed up by a gentle current. Then must 

 have come a fresh volcanic disturbance and deposit of ashes, 

 with a current moving from left to right, which broke up this 

 semi-liquid material into what might be compared with breaking 

 waves, some of which were permanently entangled in the ash, and 

 others carried away. This not only shows the original character of 

 the deposits, but also roughly the time that elapsed between the 

 disturbances. Very fine-grained material does not remain in this 

 semi-fluid condition for more than a few weeks ; and therefore we 

 have permanent evidence that, in some cases, the volcanic dis- 

 turbances were separated by only a short interval. Other specimens 

 indicate much longer periods, the breaking-up in similar cases being 

 comparatively small. 



The next illustration (PL XV) is of a case where the current 

 set in and soon increased to probably about 9 inches per second, 

 being able to develop ripples, yet not strong enough to drift along 

 any but the finer material. The rate of deposition can be learned 

 from the central portion when the current was at its maximum. 

 The ripples were about 3| inches long, of normal height, and the 

 thickness of the tails of the drift, corrected for bedding and 

 cleavage, is -86 inch. Then, in accordance with what I have 

 explained in connexion with ripple-drift, the period of the ripples 



2 "52 

 would be 3-0 x 2-59x7x-86 ~'^^ minute, in which time -86 inch was 



deposited. This is equal to about 1| inches per minute of the rock 



