by Professor Studer. - 167 



quently reverted to the same subject. Both these geologists 

 are inclined to attribute tlie cleavage of rocks, as Forbes 

 does that of glacier ice, to a pressure applied at right angles 

 to the planes of cleavage, and a contemporaneous motion 

 which has existed in the direction of those planes ; and the 

 cause which they assign for the pressure is the protrusion of 

 masses of rock from the interior of the earth. Sharpe, 

 therefore, believes he can shew that the surfaces of cleavage 

 form, over the axes of elevation, cylindrical arches, which 

 are extended for great distances with much regularity, and 

 are uninfluenced by the frequent contortions of the stratifi- 

 cation ; and also that, when two such arches intersect one 

 another, the fan-like structure is produced, which the flanks 

 of the arches are in many districts observed to possess. The 

 rains, so frequent in the west of England, much to my re- 

 gret, prevented me from obtaining more than a very super- 

 ficial view of these phenomena, of which the importance, for 

 our Alpine geology, is so great: yet I convinced myself 

 completely of the reality and general occurrence of the dis- 

 tinction between the cleavage and the stratification which 

 had previously been pointed out by Sedgwick in North 

 Wales. In the extensive slate-quarries at Bangor, in which 

 about 2500 workmen are employed, the cleavage is vertical, 

 while the strata, about a fathom thick, lie almost horizontal. 

 A similar structure is still more distinctly recognised in the 

 neighbourhood of Capel Cerig. In another respect, also, did 

 the slaty structure in Wales appear to me to difl^er from that 

 which occurs in our mountains. The cleavage there is fre- 

 quently not at all perceptible in the rock, and the splittings 

 are obtained only by the hammer. On this account the 

 stone is employed not merely for roofing, but also for sculp- 

 tures and architectural ornaments, tombstones, chimney- 

 pieces, and other objects. The structure, in fact, closely 

 resembles that of a crystal. In our slates, on the other 

 hand, the laminated divisions are always distjnct ; and the 

 employment of the material for rounded sculptures, or for 

 those of large size, would be impossible. The explanation 

 of the fan-like structure, by the intersection of two cylindri- 

 cal arches, can have no application in regard to the structure 



