202 



HARDWICKE'S SCIENCE-GOSSIP. 



fissure or saddle-joint is directed N. E. b E. (mag.) 

 or very nearly that of the lodes of the district. The 

 cleavage-planes on one side of the axis are clearly 

 not parallel to those on the other side of it ; the two 

 sets in fact form very acute angles with each other, 

 but angles much less than those formed by the sides 

 or " wings " of the saddle. The arrows show the pro- 

 bable direction of the fissure which produced this 

 local contortion. They may be said to represent the 

 resultant of the forces which folded the beds of clay- 

 slate into such sharp curves. Indistinct lines of 

 bedding are traceable in the hard beds, and in these 

 beds no cleavage planes are visible, so that the 

 phenomenon of refraction is not represented. The 

 cleavage-planes j near the crown or "buckle," form 

 acute angles with the laminae, but, lower down, the 

 two sets of planes are almost identical in dip. The 

 outcrop of the saddle — owing to denudation — is, of 

 course, irregular, but it can be traced about 24 feet 

 in vertical height. In one spot the beds appear to 

 have been folded in a very regular manner. Fig. 

 121 shows a very hard bed made up of a number of 

 thin laminae, and apparently uncleaved. From A to 

 B is only 8 inches, and, notwithstanding] this sharp 

 bending, the crown of the arch is unbroken. "When 

 a lamina is broken off and examined, it is seen to be 

 by no means uniformly curved. The top of the 

 curve is a wave whose crest and sinus form acute 

 angles with the plane of the saddle-joint. Hence it 

 would appear that oblique compression had pro- 

 duced these contortions. Is not this small unbroken 

 curve an evidence that the rock was rendered some- 

 what plastic by the intense pressure brought to 

 bear on it ? M. Tresca has shown that a powerful 

 compressing force will produce an internal motion 

 among the particles of solid metals closely analogous 

 to that of fluids, and some geologists are ready to 

 admit that locally the same thing may have occurred 

 in rocks, when they were subjected to great lateral 

 pressure.* 



Variation in the dip of cleavage-planes is still 

 more marked in our next example. Fig. 122 is 

 sketched from a series of beds occurring in the 

 Lower Llandovery (b A ), rocks of Tregaron (Q.S. 

 57 S.E.) About half a mile N.E. of the town, there 

 is the word "Castell" on the map; the beds are 

 here marked contorted. I counted five saddles and 

 four troughs in a horizontal length of only eight 

 yards. A few feet above these greatly contorted 

 beds, the clay-slate is moderately regular, and made 

 up of smaller curves or rock-ripples. Within a 

 distance of a few feet, the dip of the cleavage varies 

 as much as 26 . At a, it is absolutely vertical ( dip 

 of beds 22 S.) ; at b, it is 72 N. (dip of beds 20= N.) ; 

 at c, it is 64 N. (dip of beds 36 N.). Close by the 

 beds dip 53 N.W. and the cleavage dips 79 N.W. 



Prof. Geikie, art. Gcol., o/>. cil. 



At d, I have endeavoured to represent one of the 

 sharper curves made up of a trough and a saddle, 

 and the appearance of the cleavage dip at this point. 

 "Where there is a trough, the cleavage on each side 

 of the trough -joint forms what may be termed a 

 cleavage-trough, which is much more acute than the 

 rock or bed-trough. On the other hand where there 

 is a saddle, the cleavage abuts on each side of the 

 saddle-ioint so as to form a cleavage-saddle, also- 

 much more acute than the rock-saddle. Assuming 

 that the cleavage-planes are perpendicular to the 

 pressure which has produced them, the arrows show 

 how the direction of the pressure, or the resistance to 

 pressure, has varied over the small area. Viewed 

 generally, the cleavage-planes passing through a 

 saddle have the appearance shown ; but Fig. 123 

 is a true representation of the relation between 

 the cleavage and the bedding. Close to the saddle- 

 joint, the cleavage-planes are parallel with it, or 

 exactly perpendicular to the crown of the bent beds ; 

 lower down, the planes form acute angles with the 

 bedding, and, still lower, both sets of planes are 

 identical in dip. A prolongation of a, a, shows that 

 they form very acute angles with /', b. 



The cleavage can only be studied satisfactorily 

 on or near the surface, because weathering action 

 has there more or less separated the planes, and, 

 owing to the unequal effects of denudation, some 

 planes are left standing, and the lower or higher ones 

 have been carried away, rendering the measurement 

 of the dip an easy task. In sections of rock 300 feet 

 below the surface, and possibly at much greater 

 depths than this, the cleavage appears in section as a 

 number of somewhat indefinite parallel lines, giving 

 the rock the appearance of a ribbon-like structure. 

 These lines are seen to traverse certain portions of 

 the lead lodes uninterruptedly. Where this occurs 

 the lodes consist of several parallel veins, and the 

 cleavage-planes are visible only in the rock or 

 " country " dividing the veins. This fact must have 

 a bearing on the origin and history of the Cardigan- 

 shire lodes. 



The general conclusion to be drawn from the 

 above observations, is that where rocks have been 

 sharply folded, the cleavage sometimes varies pretty 

 considerably in dip, even within a distance of a few 

 feet, and that such variation is evidence that the 

 folding has not been of such simple nature as in 

 certain experiments, e.g. those of Sir James Hall. 

 Rocks are of very variable thickness, hardness, 

 chemical composition, &c. ; and our experiments, as 

 yet, only bear a faint resemblance to what has taken 

 place in Nature's great laboratory. But if they do not 

 teach us the whole truth, they show us the direction 

 in which the truth lies ; and better experiments 

 and keener observations will, no doubt, in time help 

 to clear up this still mysterious and fascinating 

 subject. 



E. Halse, A.R.S.M. 



