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



is not however, necessary to suppose that these effects were produced in a 

 short lapse of time. In speculating on the time required for the completion 

 of these phsenomena, we are free from any unnecessary limitations. 



There is, however, at first sight, a difficulty in comprehending the vastness 

 of those forces which nature must have applied in producinj^ effects like those 

 above described. But difficulties of tiiis kind ought to i)e little thought of, if we 

 can resolve them into any known mode of material action. Now, in a case of 

 crystallization there is something- like a definite polarity in each particle, by 

 which it is compelled to turn in a given direction, and group itself with other 

 particles in definite forms. And if this modification of internal structure be 

 carried on through a very large mass of matter, is it not probable that there 

 is an accumulated intensity of crystalline action in each part; so that the 

 whole intensity of crystalline force modifying the mass, is not equal to the 

 sum of the forces necessary to crystallize each part independently ; but is some 

 function of that sum, whereby it may be increased almost indefinitely ? I see 

 nothing improbable in this kind of accumulated attraction, and it will explain 

 many geological pha^nomena. Limestone formations, for obvious reasons, are 

 very often crystalline; but in England they are so much subdivided by beds 

 of shale and sandstone, that each part has been left to its own forces of aggre- 

 gation, and the strata are seldom obliterated. In the Alps, on the contrary, 

 the calcareous zones are generally of enormous thickness, and comparatively 

 uninterrupted by foreign matter ; and the structure of each portion is so 

 modified by the general crystalline structure of the mass, that the traces of 

 original stratification and sedimentary origin are very often entirely de- 

 stroyed. 



In the most striking cases of slaty cleavage, since the effects produced 

 through spaces of great extent are nearly uniform, the crystalline forces must 

 have been nearly uniform, at least as to certain directions, which may be re- 

 garded as a species of resultants from an indefinite number of local attrac- 

 tions. This seems to imply a certain degree of homogeneity in the masses 

 acted on ; and as a matter of fact, where the slaty cleavage is very perfectly 

 brought out, the structure of the rocks always makes an approach to homo- 

 geneity. Where the quartzose beds of coarse greywacke abound very much, 

 the cleavage is seldom very perfect, or is at least chiefly confined to particular 

 strata. And where the coarse beds predominate (as, for example, in some 

 parts of the western chain of Merionethshire, and in the east end of the Lam- 

 mermuir chain of Scotland), the slaty structure almost entirely disappears. 



All the foregoing remarks in this division of the paper relate to slate rocks, 

 subordinate to formations, parts of which are obviously sedimentary and me- 



