312 W. 21. Hatchings— Clays, Shales, and Slates. 



It has been shown that chemically such clays and shales are very 

 exact counterparts of quite typical clay-slates and phyllites. If we 

 compare the average analysis of a series of the one, with that of 

 a series of the other, we cannot point out any marked, or in any 

 way essential, difference whatever, excepting that the combined 

 water in the slates is less than in the clays and" shales. 



Again, a minute examination of these clays and shales under 

 the microscope, in thin sections and in the products of fractional 

 levigation, has led to the conclusion that mineralogically, also, there 

 is a great resemblance to the slates, inasmuch as they consist 

 mainly of a micaceous mineral. And a careful study of this 

 micaceous mineral, combined with the fact that in these finer 

 portions of such deposits there is a disappearance of all the felspar, 

 practically all the biotite, and a large part of the original clastic 

 white mica (all of which can be proved to be in abundance in the 

 accompanying coarser beds), leads to the recognition of the nature 

 of this micaceous mineral as a new formation in the clays and 

 shales, since they were deposited. 



In-comparing such clays and shales with most slates and phyllites 

 there is, however, one difference to note, and it is a matter of much 

 importance in following out the history of such beds. It consists in 

 the fact that whereas in the slates and phyllites the micaceous mineral 

 is accompanied by a good deal of a chloritic substance, with which it 

 is more or less intimately interwoven, we do not see this in the clays 

 ^ and shales. 



It was pointed out in an earlier paper (Geol. Mag., June and 

 July, 1890) that chlorite does not result from the decomposition of 

 brown mica in these deposits. A certain small amount of it may be 

 seen in them, especially in the rather coarser portions, but it is easily 

 recognized as clastic, and is doubtless due to the decay of biotite in 

 the original rocks whose waste has supplied the material for these 

 beds. In the processes of chemical interaction which have taken 

 place in these early clays and shales, chlorite has not been formed; 

 and we are justified by all that we can ascertain, chemically and 

 microscopically, in concluding that the micaceous mineral which 

 does result contains within it those components — magnesia, ferrous 

 and ferric oxides, alumina — which would be capable of combining 

 with silica on their own account to yield a chlorite. We have, in 

 fact, in these clays and shales a micaceous mineral which is very 

 complex in its nature, is highly by d rated, and does not correspond to 

 any definite species of mica. It is of a pale-green, or yellow-green 

 to yellow colour, and does not polarize quite as vividly as do minute 

 flakes of muscovite. 



In the beds of the Coal-measures in undisturbed bands (not 

 connected directly with coal-seams), suitable sections of clays and 

 shales show that the micaceous mineral is very nearly all flat in one 

 plane ; but in some of the more indurated, aud what might be called 

 more advanced, samples examined this is not so completely the case. 

 The great bulk of it is still flat in one plane, in which it gives, in 

 polarized light, a dim speckled field ; but this field is crossed by 



