218 ORIGIN OP THE ROCKS AND SCENERY OF NORTH WALES. 



ments have a beautiful opaline appearance, while others closely resemble 

 common vein quartz. They show the usual evidence of water action, and 

 were certainly transported to the position where the rock was formed, 

 not its present position be it remembered, by the agency of currents. 

 The finer grits which require the agency of a magnifying glass or micro- 

 scope to enable us to distinguish their constituent particles, give still 

 more conclusive evidence of water action. They show distinct traces of 

 banding or lamination, and here and there the most beautiful indications 

 of ripple drift. The most interesting case of this occurs in a quarry, 

 about half way along the lake and on the northern side. The rock is a 

 fine banded grit. The ripples are exposed in section, and are about two 

 inches from crest to crest, and three quarters of an inch high. Owing to 

 the peculiar character of the section, we are able to study the process 

 which led to the formation of the ripples ; the particles which form them 

 are seen to be arranged in layers running roughly parallel with that slope 

 of the ripple which points away from the direction in which the current 

 flowed. At present the dip of the beds is seen to be about 55° to the 

 W.S.W. If we suppose the beds to have been simply tilted, we may 

 infer from the structure above described, that the current flowed from 

 the S.E. or thereabouts. The same banded structure may be found in 

 the finer grits and gritty slates, and in all cases the origin must have been 

 the same. The alteration in colour and size of the constituent particles 

 must be regarded as evidence of alteration in the strength and direction 

 of the currents transporting the sedimentary material. I was particularly 

 struck with the regularity of this banded aspect. Let me read some 

 extracts bearing on this point from the notes I made on the spot : — 

 " Just before the line of shore bends round to the right to form a small 

 bay, a greenstone dyke, four yards wide, runs right up the face of the cliff , 

 altering the purple slate through which it passes for some little distance 

 from the junction. Beyond the bay are some curious banded slates. They 

 are greenish grey in colour, banded with narrow dark green strata ; 

 these bands are about two inches apart, and occur with marked regularity 

 throughout a thickness of many yards. Further on is a banded grit, 

 passing occasionally into a conglomerate. In some places the bands of 

 finer material are about three-quarters of an inch thick, and separated 

 by gritty beds of about the same thickness." 



Why do these bands succeed each other with such marked regularity ? 

 Evidently on account of some periodical change in the nature of the 

 current. Does the space between similar bands represent the interval 

 between two tides, a year, or a series of years ? If we could answer this 

 question we should then be able to determine the time represented by the 

 series of banded slates and grits with just as much certainty as we can 

 tell the time taken to form a block of wood by counting the rings of 

 annual growth. 



The next rock claiming our attention is one that forms a very 

 large portion of the surface of North Wales, viz., slate. It is extremely 

 fine in texture, the separate particles being too minute to be distinguish- 

 able by the eye, even by the aid of a hand glass. The whole mass has 



