318 W. M. Hutchings — The Origin of some Slates. 



Both the questions — why never any biotite, if we are supposed 

 to have in these slates only consolidated and cleaved original 

 deposits? — and whence all the evenly diffused rutile? 1 — appeared to 

 ine to need explanation till I examined less advanced stages of 

 presumably similar deposits. 



In many Welsh slates epidote is abundant, and in some of tbem 

 just such clusters of plates are seen as I have above described. In 

 others it is more often in larger, more solid lumps. We might 

 expect great pressure to frequently solidify such groups of plates and 

 grains ; and of course epidote may be present from other sources 

 than original biotite. 



The base and main constituent of all these slates is a small- 

 grained mica, mostly lying flat in the plane of cleavage of the rock. 



I do not think it would be going beyond what is warranted if we 

 were to conclude that markedly rutiliferous slates had resulted from 

 deposits originally containing much biotite; that slates showing 

 these platy pieces of epidote had formerly contained good large 

 flakes of biotite ; while' slates containing little or no epidote, no 

 coarse-grained quartz, felspar, etc., but rich in rutile, were formed 

 from such fine deposits as we see in the finest, of the fireclays. 

 Again, slates with but very little or no rutile may have resulted 

 from deposits free, or almost free, from biotite ; but when grains of 

 quartz, etc., indicate that the deposit was more or less coarse-grained, 

 it is still possible that biotite was present in large flakes, which, as 

 we saw, do not give rise to rutile directly ; and if epidote were seen 

 in the forms described, this supposition would receive considerable 

 support. 



The other things seen in the clays and shales are still to be found 

 in the slates ; the small plates of micaceous ilmenite (sometimes 

 here in perfect hexagonal form), the numerous crystals of tourmaline, 

 etc., etc. 



Many of the Welsh slates are much more ferruginous than any of 

 the fireclays, whether by original deposit or subsequent infiltration, 

 and this ferric constituent, under dynamic action, has resulted in the 

 countless transparent red scales of specular iron which give the 

 colour to the various red, purple, etc., slates. Others are as free 

 from iron as any fireclay of the Coal-measures, and all the Cornish 

 slates are remarkably free from it. 



A large part of the Welsh slates contains numerous original 

 quartz-grains of various sizes, and felspar may be still recognized 

 in some cases, fresh enough to give the optic figures. Small grains, 

 both of quartz and felspar, are seen in some slates to be blending 

 away into the surrounding mass. 



1 "We are indebted to Eenard (" Becherehes sur la composition et la structure des 

 phyllades Ardennais," Bulletin du muse'e royal d'histoire naturelle de Belgique, 

 1882-1883), for a series of very careful analyses of phyllites, many of which very 

 closely resemble those of North Cornwall. 



In ten analyses the titanic acid present varies from 2-28 p.c. to 0'13 p.c, the 

 average being 1-20 p.c. 



In the same analyses magnesia ranges from 2-35 p.c. to 1-13 p.c. Combined 

 water is mostly between 3 and 4 p.c, the maximum (unusual) being 4-94 p.c. 



