348 W. M. Hit tellings— Clays, Shales, and Slates. 



sufficiently extended investigation of slates, over many and largo 

 areas, would show us that there is a form of mineralogical and 

 structural development which, even in its very beginnings and e;trly 

 stages, could be sharply distinguished from anything which could 

 be produced either by dynamic action, or by depth-conditions, or by 

 a combination of these causes, operating on the same original 

 materials. If such a distinctive development could be recognized 

 and established, it would obviously be of very great value in 

 geological work. But, equally obviously, another great difficulty 

 comes in, which I can best illustrate by an example. 



I have before me a rock which represents an extremely and 

 unusually high development of the kind to which I allude. All the 

 mica is muscovite in fine large crystals, lying in all directions. 

 Not a trace of impure original mica is left. Chlorite is largely 

 developed, and is to a considerable extent as well crystallized as 

 the mica. 



There are the usual accessory constituents of highly-altered sedi- 

 mentary slates. There is no parallel structure now remaining ; the 

 rock may have been sheared to any extent before this crystalline 

 development took place, but shearing did not accompany the 

 development, and it has not suffered at all from such action since. 



But if it had suffered shearing in a sufficient degree, at a later 

 period, all trace of the present structure would be destroyed ; the 

 rock would be a perfect muscovite -chlorite schist, with these 

 minerals rolled out and arranged more or less parallel. 



It might then be claimed as a dynamically-produced rock ; and 

 though this would be quite true of its structure, it would be quite 

 untrue of its mineralogical development, with which the shearing- 

 would have had nothing whatever to do. This is probably the 

 history of a great number of schists and allied rocks, which have 

 been derived from ordinary slates by other causes and afterwards 

 sheared. It is this obvious danger of a double causation being 

 wrongly ascribed to one source only, which makes it so important 

 to investigate carefully, whenever possible, all cases of separate 

 action, so far as any really separate action has ever taken place. 



In studying the alterations in mineralogical and structural con- 

 dition caused by igneous intrusions, it is not at all necessary to 

 limit our observations to the larger manifestations of contact-action 

 around masses of granite or allied rocks. On the contrary, basic 

 intrusions of less magnitude will often serve even better to bring to 

 notice some of the particular points we have been considering ; and 

 of all the rocks I have examined in this connection, none have been 

 more instructive, or clearer in their indications, than a large series 

 of Lower Carboniferous shales altered by the intrusion of the Whin 

 Sill. 1 



In these we can see what effects have been produced on such 

 simple shales as I have described in this and former papers. We 



1 These rocks form part of a large number collected by Mr. E. J. Garwood during 

 a long period of detailed work on the geology of the" Whin Sill in Durham and 

 Northumberland. 





