﻿188 DK. H. C. SORBT ON THE APPLICATION OP [May 1908, 



stances, but they may probably be looked upon as a fair average 

 from which, to determine approximately the depth : the result being 

 that when the sand is drifted the depth is t, and when deposited 2 1. 

 However, since the bottom past the slope would be somewhat 

 protected by the slope, the possible depth in the two places may 

 be 2i and 3^. These depths seem small, but, so far as is known, 

 they are of the right order of magnitude. In any case cl-\-t must be 

 decidedly greater than d, which means that d is not great compared 

 with i, and may be small. 



Application to Particular Rocks. 



I made many observations in the Great Oolite near Bath. In 

 one good case near Box were two drift-beds, each about 15 feet 

 thick, drifted from nearly opposite quarters. These were separated 

 by a bed full of borings showing little evidence of current. In 

 other places were beds only 1 to 3 feet thick. My data are 

 imperfect, since I did not determine the angle of rest ; but they 

 indicate a depth varying from a few feet up to perhaps 20 or 30 

 feet, or more in places where there is no evidence of depth. 



I have records of very many measurements in the Millstone Grit 

 near Sheffield, and the general conclusion seems to be that the water 

 was of extremely-variable depth, and generally shallow. Thus at 

 Bell Hag the true an^le of rest, as altered by consolidation, is 25°, 

 and the rack is now .70 per cent, of its original volume. With this 

 angle are beds 25^ and 5 inches thick, 20° and 36 inches thick, 

 and 17° and 15 inches thick, which can be explained approximately 

 by supposing that in one part or another, before and after depo- 

 sition and filling up, the depth varied from 12 to 18 feet down to 

 1 or 2 feet, with a general average of 6 to 10 feet. 



General Conclusion respecting Sandstones, etc. 



The facts now described enable us to divide sandstones and 

 analogous rocks in the following manner : — 



1. Thinly- or thickly-bedded rock, without ripples or drift-bedding, and 

 showing little or no graining of the surface in the line of the current. Good 

 examples of this occur in the Old Red Sandstone of the Black Mountains, 

 in the Llanthony valley. This could be explained by supposing that the 

 water was at considerable depth, and the material mainly deposited from 

 above, not drifted along the bottom, where the velocity of the current was 

 much less than 6 inches per second. 



2. Thinly-bedded rock, with well-marked graining of the surface in the line 

 of the current, indicating a mean velocity varying up to about 6 inches per 

 second, but showing few or no ripple-marks. 



3. More or less thick masses of rock almost entirely made up of ripple-drift. 

 This must have been when the velocity of the current was something like a 

 foot per second, but varying with the character of the sand, which drifted it 

 along the bottom accompanied by more or less rapid deposition from above ; 

 and, as the ripples advanced, more was deposited on the sheltered side than 

 was washed up on the exposed side, so that the rate of deposition would be 

 known if our knowledge of the advance of ripples were more complete. But, 



