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



I suggested, tentatively, that some of the "spots" were possibly 

 due to portions of this solution, which, in the outer portions of the 

 contact-aureoles, "would tend to draw together as little spots and 

 patches," and so consolidate among the other ingredients of the rock. 

 1 still think that this, to a considerable extent, expresses the state 

 of the case ; but that it needs a little amplification. These spots, 

 although in the outer contact-areas, do not occur except where 

 a decided amount of recrystallization is already taking place. This 

 recrystallization, as we have seen, consists largely in a development 

 towards muscovite, with a rejection of the chloritic constituents of 

 the original micaceous mineral. These constituents pass over into 

 the solution which permeates the rock, and are themselves, in these 

 cases, deposited out of this solution, and with residual portions of 

 it, in the manner suggested to form the " spots." 



I formerly desci'ibed in detail the nature and appearance of many 

 of these "true spots" in several occurrences. They are, in fact, 

 due simply to another form of the concurrent separation from the 

 slate-mica of purer mica and of chloritic bodies. Like the inter- 

 woven chloritic matter, they vary in all degrees of green and yellow 

 tints, and some, again, are almost colourless in thin sections. They 

 vary also in degree of optic activity in a similar manner ; we get 

 spots of true chlorite, and we get all gradations from this of 

 apparently quite indefinite compounds. 



The frequent aggregation in the spots of iron-ores, of ilmenite- 

 flakes, of rutile, anatase, etc., is to a large extent part of the same 

 process of separation and recrystallization of formerly combined 

 materials. 



In my former notes I urged that it was desirable to make a strict 

 separation between " true spots " and certain indefinite grains, or 

 imperfect crystals, of particular minerals. But this opinion I would 

 now modify and partly withdraw, after more extended observation. 

 One sees so many cordierite grains which exactly resemble spots 

 in mode of occurrence ; and here and there one sees such clearly 

 apparent cases of gradual transition from chloritic spots into definite 

 cordierite, that a strict separation is not practicable. The passage 

 from a chloritic substance to cordierite does not require anj' qualita- 

 tive change. It only needs the removal of water, and a certain 

 degree of molecular interchange with a permeating solution; and 

 thus we so often see the occurrence together of chloritic and 

 cordieritic spots, or the giving way of the former to the latter as 

 the contact-action increases in intensity. 



Similar considerations apply to staurolite, but the transition is 

 not, in any case that I know of, so apparent ; nor does this mineral 

 occur nearly so frequently as cordierite. 



Again, spots of chloritic nature pass over in a similar manner into 

 biotite, and here also no line can be drawn ; — the biotite is often as 

 truly a spot as is the chloritic substance. 



We may thus satisfy ourselves, by examination of a sufficient 

 number of contact-slates, that here, again, the main change in them, 

 in their earlier stages, before we come to the more intense actions 



