W. M. Hutchings — Clays, Shales, and Slates. 313 



little veins and strings and wider bands, highly inclined or even 

 vertical to this plane, of a minutely-felted and wavy mass of flakes 

 of the same or a similar mineral, but obviously in a more developed 

 stage. There is no mistaking that these veins and bands have been 

 formed later than the main mass seen in the section, and that 

 they represent a further development and crystallization of a 

 micaceous mineral which is still in general nature and appeai'ance 

 the same as the main mass, though with ci'ossed nicols it shows 

 more vivid tints ; and the peculiar felted and wavy structure of the 

 little veins is very noticeable. 



If we now pass at once to the consideration of what we may call 

 fully-developed slates of Silurian or Cambrian age, limiting ourselves 

 for the present to occurrences concerning which there is no evidence 

 that they have ever been affected by igneous intrusions; and if we 

 compare these rocks with the clays and shales we have just left, 

 we find, as before pointed out, only one chemical difference, viz. 

 the decrease in the amount of combined water. But careful minera- 

 logical study of the slates with the microscope results in our being 

 able to note two main points of variation between them and the 

 clays and shales. The degree of these differences varies very much, 

 as the exact condition of the slates varies in different occurrences, 

 but in nearly all cases we can see the points in question. There is 

 a higher development of the micaceous mineral, and concurrently 

 there is the separation in the rock of a chloritic substance, to which 

 reference was made above. 



By higher development of the micaceous mineral is meant, not 

 only a development in size, for this is sometimes not at all striking, 

 but an advance towards the nature of muscovite, often with a notice- 

 able clarification as to colour, and increase in the vividness of 

 polarization-tints. 



The chloritic mineral which has been formed at the same time 

 is very intimately interwoven into the texture of the i - ock. Often 

 it is mainly in minute mixture with the mica, flake for flake as 

 it were, but usually this form of occurrence is accompanied by 

 the formation of streaks and lenticles of chlorite, in which mica 

 is seen to a much less extent, or from which it is even absent. 



It seems quite evident that what has taken place in the rock 

 during its progress from a clay or shale, has consisted mainly in 

 this further development and crystallization of the micaceous 

 mineral, during which it has rejected some of the components of 

 the original more complex combination, which components have 

 then given rise to the formation of the new chloritic mineral. 



This chloritic mineral varies in its nature and appearance according 

 to the chemical composition of the rock in question, the principal 

 condition being, apparently, the amount of iron present, and more 

 especially that portion of it which is in the form of ferrous oxide. 

 Thus, sometimes the mineral is dark green and very dichroic, with 

 decided "polarization" — is, in fact, chlorite, andean be proved to 

 be such, — while in other cases it is very pale in colour and optically 

 very faint ; and in some few rocks it is, in the thin sections used, 



