244 



HARDWJCKE'S SCIENCE-GOSSIP, 



hammer, we can in time perhaps, succeed in breaking 

 it along these lines — in other words the rock is to 

 a certain extent cleavable along these lines ; but in 

 nineteen cases out of twenty, the rock will fail to split 

 along the latter, thei-efore the cleavage is in a very 

 cnide or undeveloped condition. We cannot fail to 

 conclude from this fact that the rock has been sub- 

 jected to a tremendous mechanical force, which has 

 produced these lines of cleavage, but that this force 

 was not sufficient to obliterate the planes of bedding, 

 and to produce distinct planes of its own. 



We notice another remarkable fact. Where the angle 

 which these lines make with the line of bedding is a 

 moderate one, say from 30° to 50°, the rock will not 

 cleave along the former lines, but breaks only along 

 the plane of bedding, and along the joints perpendi- 

 cular to these planes ; but where the lines of cleavage 

 make a high angle with the beds, say 75°, the rock 

 cleaves along the lines of cleavage,* being broken in 



of iron-pyrites, we must conclude that their contents 

 were deposited from solution, and that the veins were 

 formed, or at all events filled up subsequently to the 

 consolidation of the clay-slate. The history then of 

 this particular quarry, and of the surrounding strata, 

 resolves itself into the following periods : (i.) The 

 beds were deposited in the first instance horizontally, 

 or nearly so. (2.) After consolidation they were 

 tilted up to the moderately high angle of 40°. 

 During this process of tilting, or subsequently to it, 

 some of the planes of bedding became opened in a 

 greater or less degree, and a series of fissures were 

 formed in the beds, some parallel to the planes of the 

 beds, and others making an acute angle with them, 

 (3). Heated water from certain depths below the 

 surface of the earth containing silica in solution filled 

 up these veins, and silica was deposited or crystallised 

 therein. Crystals of iron-pyrites were afterwards 

 deposited on the quartz, either in clusters, or in more 



Fig. 129. 



Fig. 140. 



Fig. 141. 



this way into rough rhombohedrons. On once more 

 inspecting the veins of quartz we notice that the 

 cleavage lines are distinctly traceable through the 

 veins themselves, especially where they are decom- 

 posed, and still so, but in a less degree, where the 

 veins are in their normal state. We infer from this 

 —and it is an important inference— that the cleavage 

 lines were produced subsequently to the formation of 

 the quartz veins. Now these latter show a "comb- 

 structure," an indistinct one it is admitted, but from 

 this fact, and from the fact of the presence of 

 numerous little vughs of rock-crystal, and of crystals 



* This observation is somewhat different from that of 

 Mr. T. McKenny Hughes, who remarlcs that "where a 

 rough cleavage cuts flagstones at a considerable angle to the 

 planes of stratification, the rock often splits into large slabs, 

 across which the lines of bedding are frequently seen, but 

 when the cleavage planes approach within about 15° of stratifi- 

 cation, the rock is apt to split along the lines of bedding." 

 (Lyell's " Student's El. Geo!." 1871, p. 573.) The latter in our 

 case do not appear across the slabs, but from the upper and 

 lower surfaces of it ; the two sides are formed by the vertical 

 joints, and the front and back by the planes of cleavage. 



Fig. 142. 



or less isolated crystals. (4.) The rocks were next 

 subjected to tremendous lateral pressure, producing 

 undeveloped cleavage. (5). Very recently atmos- 

 pheric influences have affected the rock, vertical 

 joints have widened, and have in places become filled 

 with rubbish ; water percolating from al)ove through 

 numerous cracks has helped to disintegrate the rock 

 and the veins, and to some extent has decomposed 

 the iron-pyrites in the latter. 



About three miles to the east of this quarry there is 

 a quarry which has been worked for slate, but which 

 has lately been abandoned for want of capital. The 

 beds here are not tilted up quite so much as at the 

 former quarry, dipping only at an angle of 35°, but 

 the strike is exactly the same, namely to the N.N.W. 

 The cleavage here is more highly developed, and 

 moreover no quartz veins are visible. Fig. 144 is 

 a sketch of the beds taken from one side of the 

 quarry. Along a are seen a series of lines somewhat 

 irregular, dipping at an angle of 76° from the horizon. 

 The lines in /', c — m are flatter, straighter, and more 

 crowded together. Again the lines running through 



