Prof. Sedgwick on the Structure of large Mineral Masses. 471 



porphyries. It is evident^ after what has been stated^ that such an associa- 

 tion wi«j/ be purely accidental *. 



Leaving- all further comparison between the structure of Cumberland and 

 Wales, I return to the description of the most general facts exhibited in slaty 

 cleavage. If we examine a quarry where this structure is well developed, we 

 find a nearly homogeneous mass, easily separable into thin parallel lamina. 

 But the thickness of these laminae is not defined by joints (i. e. by fissures 

 at definite distances) ; for the cleavage of each part may be carried on inde- 

 finitely, or at least so far as the operation is not interrupted by a mere mecha- 

 nical difficulty. That this arrangement is crystalline it is impossible to doubt, 

 when we examine the planes of cleavage, and see them coated over with 

 flakes of chlorite and semicrystalline matter, which not merely define the 

 planes in question, but strike in parallel flakes through the whole mass of the 

 rock. Were there any doubt of this conclusion, it is further confirmed by the 

 fact, that these planes of cleavage are inclined at various angles to the planes 

 of stratification, and are, perhaps, in no instance coincident with them. This 

 last fact is of great importance, and is now generally admitted by English 

 geologists. But it requires to be made still more prominent; for it is not con- 

 sidered in its proper extent by Continental geologists, and by some of them 

 is actually denied. 



Again, some English writers do not always distinguish between a jointed 

 and a slaty structure ; and even Dr. M'Culioch (whose general accuracy in 

 the description of primary rocks is above all praise,) seems to consider a lami- 



* There is a still lower system of schistose rocks in North Wales (not noticed in the preceding 

 account), which are widely expanded in the Isle of Anglesea (Professor Henslow, Cambridge Phil. 

 Trans., vol. i. p. 565), and occupy the south-west coast of Caernarvonshire from Porthdinlleyn to 

 Aberdaron. They are chiefly made up of chlorite schist, sometimes passing into mica schist ; and 

 subordinate to them are calcareous portions, passing on the one hand into a beautiful white sac- 

 charoid marble, and on the other into vert antique and serpentine. There is also a lower group 

 in Cumberland, which contains no calcareous beds, and hardly any calcareous matter ; and passes 

 in its inferior portion into a rock abounding with chiastolite ; and more rarely into a quartzose mica 

 slate, and into a rock which (perhaps improperly) has been described as gneiss. These two lower 

 groups have few analogies of mineral structure, though they seem to occupy nearly the same geo- 

 logical place, and they have no organic remains. 



Perhaps some of the lower schists of Cornwall may be on the same parallel with these two 

 groups, however unlike them in structure and composition. It perhaps deserves remark in this 

 place, that the lower schists of Cornwall, though certainly much changed since their deposition, 

 are very distinctly bedded. Near the granite, besides the stratified, they often exhibit a beauti- 

 fully jointed structure ; but in hardly any instance a true slaty cleavage, in the sense in which those 

 words are used in this paper. 



3 P 2 



