W. M. Hatchings — Clays, Shales, and Slates. 347 



of the inner zones, is based upon this same tendency of the micaceous 

 mineral to take higher development and to reject some constituents ; 

 and that to these rejected constituents is due the formation of 

 chlorite and allied bodies (often taking the form of "spots"), 

 of biotite, of cordierite, and of staurolite. And we may note that 

 even in very moderate degrees of contact-action, we can get effects 

 which are not in any way approached by those of some very intense 

 manifestations of dynamic action. 



As regards the third agency for the crystalline development 

 of slates, it is not possible to say much, because direct evidence of 

 any value is practically absent, excepting what is rather on the 

 negative side. That is to say, we have plenty of cases in which 

 we know that rocks have been at one time under enormous 

 thicknesses of cover, so that if simple depth-conditions could bring 

 about the sort of crystalline development we are considering, 

 we might look for its very marked appearance ; yet we do not see 

 any striking degree of it, but only such very moderate stages as we 

 have already noted in some of the sheared slates. 



On the other hand, we have rocks in which such development 

 is carried to a very high point, in a manner closely paralleled in 

 contact-areas, but without any proof at all of contact-action, and 

 with strong evidence in the rocks themselves that this development 

 was not accompanied by crushing and shearing. We cannot here 

 deny the possibility that what we see is the result of simple depth- 

 conditions ; but it is quite open to us to say that a much more 

 probable cause is the action of concealed igneous masses ; and, 

 indeed, this view seems rather the more rational, having regard 

 to the strong evidence for the very moderate effects of depth- 

 conditions, either alone or together with dynamic action. 



We can only leave this question an open one, awaiting further 

 evidence; and simply note the fact, that where we have these cases 

 of high development, not directly accounted for, they are always on 

 the same lines exactly as in contact-areas, and are often plainly 

 not dynamic. 



The main point important to emphasize in all such observations 

 appears to be that both in contact-development, and in this develop- 

 ment which we cannot definitely ascribe to contact-action, we 

 always get more or less of a decided advance to muscovite ; 

 whereas, in so many cases of intense dynamic action, we see that 

 the micaceous mineral is not nearly so far advanced. There has 

 been some progress, but it does not seem to be able to go beyond 

 very moderate limits, and it leaves the slate with an impure mass 

 of felted mica, of complex nature, among which no well-marked 

 separate flakes and crystals have begun to develop themselves. 

 Moreover, even this very moderate degree of progress we cannot 

 safely place to the credit of dynamic action, as depth-conditions may 

 have caused it. And, indeed, in other cases not dynamically affected, 

 we may see that just such a moderate crystalline development has 

 taken place on much the same lines. 



It may be, and it does not seem at all improbable, that a 



