Prof. Sedgwick on the Structure of targe Mineral Masses. 477 



cleavage ])Ianes dip nearly north-west, at an an<;Ie of about 48°. The slaty 

 cleavage alTects the limestone beds, which only break freely in that direction. 

 The half-jointed and half-fissile structure of the limestone beds is therefore 

 due to the same cause which produced the true fissile structure in the beds 

 above and below. 



Plate XLVII. fig. 8. 



We have here the profile of a ridge below Maes Maillyon lime quarry, a 

 few miles south-east of Bala. In a succession of low knolls nothins" is at first 

 seen but the cleavage planes dipping west-north-west. The slates are, how- 

 ever, striped; and the stripes dip east-south-east, as represented by the dotted 

 lines. Were there any doubt about the true dip, it is set at rest by the beds 

 of limestone and calcareous schist, full of organic remains, on the left side of 

 the section, which are parallel to the dotted lines ; yet the cleavage passes 

 through them, and affects the fracture of thick masses of crystalline limestone. 



The preceding sections are sufficient for my present purpose, and they are 

 given, not as exceptions to, but as examples of, the general structure of the 

 regions from which they are taken. A rugged country, more than thirty 

 miles in length, and eight or ten miles in breadth, stretching from the o-or^e 

 of the Wye, above Rhaiadr, to the upper gorges of the Elan and the Towy, 

 and to the hills west of Llandovery, exhibits, on a magnificent scale, thousands 

 of examples like figg. 4. and 4 a. The whole region is made up of contorted 

 strata; and of the true bedding there is not the shadow of adoubt. Many parts 

 are of a coarse mechanical structure; but subordinate to them are fine, crystal- 

 line, chloritic slates. But the coarser beds and the finer, the twisted and the 

 straight, have all been subjected to one change. Crystalline forces have re- 

 arranged whole mountain masses of them, producing a beautiful crystalline 

 cleavage, passing alike through all the strata. And again, through all this 

 region, whatever be the contortions of the rocks, the planes of cleavage pass 

 on, generally without deviation, running in parallel lines from one end to the 

 other, and inclining, at a great angle, to a point only a few degrees west of 

 magnetic north. Without considering the crystalline flakes along the planes 

 of cleavage, which prove that crystalline action has modified the whole mass, 

 we may affirm that no retreat of parts, no contraction of dimensions in passino- 

 to a solid state, can explain such phaenomena as these. They appear to me 

 only resolvable on the supposition, that crystalline or polar forces acted on the 

 whole mass simultaneously, in given directions, and with adequate power. It 



VOL. III. SECOND SERIES. 3 Q 



