4S2 Prof. Sedgwick on the Structure of large Mineral Masses. 



crystalline parts; but we look in vain for any such arrangement. The sphe- 

 roidal blocks into which the Tors decompose^ and the concentric crusts which 

 exfoliate on their continued disintegration, are the chief facts in favour of the 

 theory ; but they may be explained as well without it. For any homogeneous 

 cuboidal mass of rock will naturally decompose into a spherical form ; and the 

 exfoliation of concentric crusts is no proof of globular concretionary structure ; 

 because ancient pillars of granite have been known to exfoliate in cylindrical 

 crusts, parallel to the axes of the pillars ; and even pillars of oolitic limestone, 

 which unquestionably have no spheroidal structure, sometimes exfoliate (e. g. 

 in the second court of Trinity College Cambridge) in crusts parallel to the 

 axes of the several pillars. To what, then, do we refer the prismatic jointed 

 structure of various granitic rocks ? We reply, chiefly to a tension on the 

 mass when it passed from a fluid to a solid state; combined sometimes with 

 those great crystalline actions which in argillaceous rocks have produced a 

 true slaty cleavage. This opinion is, I think, fortified by an examination of 

 the prismatic sandstone obtained from linings of old furnaces. These sand- 

 stones have been intensely heated, and sometimes partially fused, and their 

 prismatic structure, transverse to the wall of the furnace, is clearly super- 

 induced by the action of heat. They have undergone alterations of dilatation 

 and contraction, which may have produced their several joints ; but we do 

 not discover in them the least germ of any globular arrangement. 



In the cases just considered, there is a clear distinction between a jointed 

 and slaty structure. Still, even in formations of true granite, we occasionally 

 see imperfect indications of a cleavage, which may sometimes have modified 

 the direction of one set of joints. This kind of cleavage is called the grain 

 of the rock, and is designated in many parts of the North of England by the 

 word "bate", where we are constantly told by quarry-men that no rock is 

 without its "bate". Now the grain has, I believe, in most cases, been pro- 

 duced during the passage of the whole granitic mass into a solid state, by 

 that kind of compound crystalline force which has produced the transverse 

 laminations of argillaceous schist. 



Every one has heard of the great vertical, parallel layers composing the 

 central mass of Mont Blanc, and of the conclusions Saussure drew from this 

 structure. On a small scale we have a somewhat similar arrangement of parts 

 in the granitic rocks of St. Austell Moor, which from one end to the other 

 are made up of alternating, parallel masses of granite and schorl rock. In some 

 places the rock is granite, striped by veins of schorl or schorl rock; in other 

 places we have schorl rock, striped by veins of granite ; and here and there 

 one rock prevails, to the exclusion of the other. The range of these layers is 



