270 W. M. Eutchings—The Origin of some Slates. 



forms a part, is everywhere seen when the sections are thin enough ; 

 and many grains of quartz and felspar, detached from the mass and 

 separately mounted, retain a skin or coating of this paste with its 

 secondary mica and the small rutiles, the impression produced being 

 that the surfaces of some such grains are really attacked, as it were, 

 by the paste and beginning to blend with it, not simply mechanically 

 adhering. 



Turning now to sections of the most fine-textured of all the 

 bands, the smoothest finest-grained clay, it is seen that the still 

 recognizable original minerals play a comparatively small part, the 

 main component being the same material which is the paste in the 

 coarser-gi'ained beds. Biotite has wholly disappeared ; original 

 muscovite is still present throughout, though nearly all very small. 

 But little epidote is seen ; all the various indeterminable matter as 

 before, with an increase in the amount of indistinct granular matter 

 usually spoken of as kaoline. The entire mass is crammed with 

 small rutile crystals and microlites, and shows all over it more or 

 less of the small flakes of secondary mica with the inclosures of 

 rutile, etc. 



Looking, finally, at the other extreme, — the coarser bands of 

 micaceous sandstone before mentioned, and the thicker beds of the 

 same overlying, — these are seen to be composed of quartz, felspar 

 and two micas as before, the biotite having been in excess. Owing 

 probably to the greater percolation of water through this coarser 

 material, the biotite has suffered more alteration, even in the largest 

 flakes, than in the other bands, a larger proportion of it being 

 wholly removed, leaving clusters of grains and plates of epidote 

 to mark its former position. Very little fine silt of any kind seems 

 to have been deposited during the conditions which obtained whilst 

 these coarser materials were being laid down. There is hardly any 

 of the rutiliferous paste to be detected anywhere, nor secondary 

 mica ; indeed, no very small mica of any kind. 



Materials such as I have attempted to describe above are obtain- 

 able all over the district in natural exposures and from coal-pits, 

 and always show the same things, as do also specimens from other 

 and distant coal-fields. 1 I have several examples mounted illus- 

 trating the passage of such shales and clays, still showing much 

 original biotite, into finer grades with no trace of that mineral 

 remaining. All the fire-clays examined, — those being worked 

 extensively for brick-making and others taken from beds not so 

 worked, — all show the same composition and structure under the 

 microscope, though varying a good deal in the relative amounts of 

 sand, original mica and " paste " ; and the part I speak of as paste 

 again varies very much as to the proportion of it which consists of 



1 Among other material examined, I am indebted to Mr. G. H. Askew, of 

 Aspatria, near Carlisle, for a series of specimens of the cores from a bore-hole put 

 down at that place to prove the coal-seams. This series of clays, shales, and 

 micaceous sandstones, extending to 800 feet in depth, proves to have been all 

 derived, like the beds in the Newcastle coal-field, from the waste of a granite with 

 two micas ; and the changes undergone by the materials of the deposits have been 

 the same. 



