﻿198 DR. H. C. SORBT OK THE APPLICATION OF [May I908, 



in its present state, or to about 2| inches when deposited ; but, since 

 the current was probably less than 12 inches per second, the rate of 

 deposition maj" be looked upon as about 2 inches per minute. This 

 may seem rather a large amount, but the facts appear to justify 

 the conclusion, which is not unreasonable in the case of a volcanic 

 eruption. The current would seem to have set in somewhat sud- 

 denly, and to have continued for about 2 minutes, after which fine 

 semi-liquid mud was deposited, somewhat broken up by a fresh 

 eruption of ashes, which, judging from my experiments, probably 

 took place in the course of a few weeks or months. 



Another specimen is very different, and shows that the current 

 suddenly increased from under 6 inches per second to 12 inches for 

 a doubtful period, but probably for about 2 minutes, during which 

 deposition took place at the rate of only '10 inch per minute. 



The third illustration (PI. XYI) is of great interest. In the 

 lower part is level bedded deposit, indicating a current too small 

 to develop ripples. This gradually increased until it was strong 

 enough to wash along the sandy ashes, and produce a thin bed 

 of true ripple-drift, deposition taking place at the rate of -4 inch 

 per minute. The current then gradually decreased, and could no 

 longer drift along the coarser part of the material, and the fine- 

 grained semi-liquid clay was deposited, and was partlyin this state 

 when a fresh disturbance brought coarser ashes and broke up and 

 entangled some of the creamy material. 



The maximum angle at which stratula dip is of much interest, 

 as indicating the character of the material when it was deposited. 

 Making all the necessary corrections, and allowing for the com- 

 pression from the condition of a newly -deposited rock to one almost 

 solid, I find that the angle of rest of the material must have been 

 closely the same as that for a fine-grained volcanic sand. Hence, 

 although the slate is now of almost exactly the same hardness 

 throughout, we have good physical evidence that one part was 

 originally fine loose sand, and the other a semi-liquid clay. 



All these results must be looked upon as only approximate, but 

 they are as likely to err on one side of the truth as on the other. 

 As will be seen, the rate of deposition varied greatly, but all my 

 specimens agree in showing that currents set in more or less sud- 

 denly, and, after continuing for a few minutes, died out. It seems 

 to me, therefore, much less likely that they were currents of the 

 kind so common during the deposition of many rocks, than due to 

 volcanic disturbances accompanying the throwing-out of the ashes 

 of which the rock is composed. It is possible that further study in 

 the quarry would show cases in which the current acted for a 

 longer period. I describe only some of the specimens that I collected 

 and prepared in a suitable manner. At all events, there seems, 

 to be here good evidence of the rate of deposition, and indication 

 of the intervals between different volcanic disturbances. 



