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



quite colourless and optically inert. For convenience we will speak 

 of it in all cases as " chloritic," without attempting to specify its 

 exact nature and composition. 



It is often difficult to make out how much of this substance is 

 present, even when it is of a decided green colour, because of its 

 very intimate intermixture and the smallness of some of its flakes; 

 and when it is verging on colourless it is even difficult to detect it at 

 all in ordinary sections. But for the study of this class of rock it is 

 of very great use to take specimens, of suitable size for cutting 

 sections, and heat them to dull redness for some time. This dehy- 

 drates the chloritic mineral, and when it is of green colour leaves it 

 with more or less deep shades of brown or of red, in which condition 

 it can be easily detected, even in a most minute and intimate state of 

 subdivision. And even when it is nearly or quite colourless, it is 

 thus rendered more opaque, becomes tinted, and can be seen where 

 it was previously invisible. Sections made from these dehydrated 

 specimens are, therefore, of the greatest value when used in con- 

 junction with those of the normal rock. 1 



They show, too, another useful and interesting point. The more 

 impure and less developed mica is also affected by dehydration and 

 by the oxidation of the iron it contains, taking on colours which 

 range from yellow to various shades of brown, and even to good 

 strong red, according to the degree and nature of impurity. 

 "Where these colours are sufficiently deep, they destroy the optic 

 characteristics of the mica ; but even when they are only very 

 pale it is seen that such mica, after heating, is much less active in 

 polarized light than when in the normal state. Thus the micaceous 

 mineral is more or less affected according as it is more or less 

 developed towards muscovite, which is not so altered by moderate 

 ignition, its colour and polarization-tints being not perceptibly 

 influenced. In this manner we are enabled to make very instructive 

 comparisons between slates in various stages of evolution. 



It is of interest to note that this test applies also to the new 

 micaceous mineral of the clays and shales. I described (Geol. Mag., 

 April, 1891) the results of some very careful levigations carried out 

 on a typical " fireclay," and stated that, in the finest parts of the 

 products obtained, a considerable proportion consisted of very minute 

 flakes of a pale yellowish or greenish mineral, most of which could 

 only be well seen when mounted in water. I consider that this 

 portion of the products largely represents the new mineral formed in 

 the clays and shales in question. 



If it be heated to redness, and then mounted in balsam in the 



1 There is no harm in here reiterating a caution as to the necessity of very thin 

 sections, this being especially important for those cut from dehydrated specimens. 

 Personally, I have never seen any made by English cutters which are of the least 

 value for studying this class of rocks. It is decidedly a case of things ' ' made iu 

 Germany" being better ! The vagueness and indistinctness of many descriptions 

 given of slates, etc., are well accounted for when one sees the sort of paving-slabs 

 which have often done duty as " thin sections," and when one sees them mouutid on 

 such thick glasses, and covered with such thick covers, that no use of higher powers 

 or proper illumination is possible. 



