IV. M. Hutchings — Clays, Shales, and Slates. 311 



question of cleavage, etc., which is of interest, as the mineralogical 

 nature and inner structure. 



In the rocks we call " slates " we are accustomed to see a more or 

 less high degree of cleavage, and, indeed, to commonly more or less 

 associate this development of fissility with the very name. But with 

 this cleavage we usually find that other changes have heen brought 

 about, which are petrologically of far greater interest and importance, 

 — great alterations in the component minerals and in their arrange- 

 ment in the rock. 



We know that the development of cleavage is a purely mechanical 

 affair, which may be brought about in any fine-grained rock, 

 sedimentary or otherwise, by pressure. In dealing petrologically 

 with " slates," we are not tied to this quality of cleavage. We can 

 have bodies of sedimentary rock around igneous masses, which have 

 had their former cleavage obliterated by the effects of the intrusion 

 of these masses, but which we must still call slates. And we 

 may have beds of soft shales, converted by such igneous masses 

 direct into contact-rocks without any cleavage at all; which are 

 mineralogically and structurally slates, and to which we could not 

 at present conveniently apply any other name. 



It is just these mineralogical and structural developments of the 

 deposits, which accompany the change into slates, which are of so 

 much interest for us to endeavour to follow out ; to find, if possible, 

 the different causes to which we must attribute the variations in 

 these developments, both in kind and in degree, which become 

 apparent to us when we study sufficient examples of slates, or going 

 further forward, of phyllites and schists. 



It may be as well to take a general review of the subject of the 

 origin and development of the deposits from which our normal 

 sedimentary slates, and allied rocks, may be traced ; and to take 

 stock of such facts concerning this development as seem, on 

 microscopical, chemical, and general evidence, to be either fairly 

 demonstrated or rendered highly probable. 



In the first place, then, all the evidence we cau gather appears to 

 justify the belief that the great bulk of these slates is derived from 

 the sediments consisting of the waste of vastly older granitic and 

 gneissic rocks, and that in the indurated clays and soft shales of 

 the Carboniferous beds we have a series of closely similar deposits, 

 of like origin, preserved for us in a relatively early stage of their 

 history. Any observations, therefore, which we can make on these 

 Carboniferous deposits, will assist us to a general understanding of 

 the progress of all such sediments into slates, and of the alterations 

 which these, again, may undergo at later stages of their existence. 



If we take for our study mainly the finer-grained parts of these 

 deposits, we shall best arrive at the desired conclusions ; because 

 any increase in coarseness of grain simply leads on to the more 

 quartzy slates and grits, and finally to the argillaceous sandstones; 

 in all of which rocks the original nature and subsequent changes 

 of the finer argillaceous portion of the materials are the main points 

 to be considered. 



