342 Prof. Hughes, On the altered rocks of Anglesea. [Feb. 23, 



we have subordinate hard unyielding beds which are incapable 

 of this kind of extension, they are broken up and discontinuous, 

 and often in the folds protruded through the layers of yielding 

 shale. 



Of this kind of action some interesting examples have come 

 under my notice in the high ground between the head waters 

 of Dent and Ribblesdale in Yorkshire. The mass of the mountain 

 consists of Yoredale Rocks made up of shales, sandstones, and 

 limestones. One of the limestones rests upon a thick bed of shale, 

 and is quarried for black marble in a small ravine near the bottom 

 of the hill. When the workmen, removing layer after layer, reach 

 the lowest beds, the pressure of the mass on either side squeezes 

 out the shale and the limestone slabs in the middle bulge up, 

 and if struck with a pick along the lines of tension are apt to 

 break and pieces fly up with considerable force. In making a 

 tunnel on the Settle and Carlisle Railway through the upjoer part 

 of the same series a similar thing was observed when too thin a 

 lloor of rock was left resting upon shale. The common phenomena 

 of creeps in coal mines, and of the bulging up of the fore-shore 

 when there has been a land slip, or the rising of the surrounding 

 peat when a bank has been thrown across a moss are examples 

 of the same kind . 



This must go on among the larger rock masses also, compressed 

 in one place or in one direction they give way in another, ex- 

 hibiting in a large way what is seen on a small scale in the 

 examples given above, and so we observe that, in consequence of 

 large portions of our older formations being made up of com- 

 jDressible mud which behaves more or less as a fluid, when the 

 great weight of ever accumulating strata is heaped up over one 

 part of it, the pressure is transmitted laterally, and the rocks 

 yield; compressed horizontally, they often rise elsewhere vertically. 



The readjustment takes place in three ways: either (1) the 

 rocks are thrown into great folds as is generally the case when 

 they are composed of large beds of varying texture ; or (2) they 

 are puckered into numerous small contortions forming a gnarled 

 or crumpled schist. This is chiefly the case when the rock is 

 composed of laminae of fine mud with thin layers of coarser un- 

 yielding matei'ial, as in the great masses of greenish schists in 

 Anglesea. Or (3) all the particles are squeezed together so that 

 those which had, or could assume, a flat or elongated form, are 

 forced to arrange themselves with their longer diameters in planes 

 at right angles to the lateral pressure. This happens in all 

 masses of homogeneous mud and produces cleavage, and of this 

 we have plenty of examples in the black slates of Anglesea. All 

 these are only different results of the same force, and from the 

 nature of the case it must be a local phenomenon depending upon 



