74 PEOCEEDIN-GS OF THE GEOLOGIC A.L SOCIETY. 



I have thought it very desirable to call attention to these facts, 

 since I think the explanation I have given removes a very serious 

 difficulty in completely explaining the mechanical origin of slaty 

 cleavage in rocks which have yielded to pressure as imperfectly 

 plastic substances. 



Slates of the Englisli Lcike-dist^-ict. 



It would be difficult to find a better illustration of the influence 

 of the mineral constitution of the original materia] on the final 

 nature of more or less altered stratified rocks than is furnished to 

 us by comparing the green slates of Cumberland and Westmore- 

 land with those from Wales already described. It is, I suppose, 

 agreed on by all of us that they were to a great extent derived 

 from volcanic ashes ; but no mere examination of the rocks in their 

 natural state would lead any one to imagine that the microscopical 

 proof of their true volcanic nature was so perfect. At the same 

 time this method of study proves equally well that often such 

 great changes have subsequently occurred that very little of the 

 original material can now be seen, and the character of the altered 

 deposit closely approaches that of a rock solidified from fusion. 



It would be interesting to ascertain the distribution in time and 

 space of the scarcely altered and greatly changed rocks ; but, un- 

 fortunately I can indicate this only in a very incomplete and 

 perhaps incorrect manner. The general appearance, even of the 

 two extremes, is not so different as to attract attention at once ; 

 and it was only by careful microscopical investigation that I 

 became aware of the existence of any considerable alteration. 

 Those of my sections which show the least change are all from 

 Langdale and Rydal. At Ambleside, Troutbeck, and Smethwaite 

 Bridge there is more alteration ; whilst at Keswick and E-osthwaite 

 it is much greater, and sometimes so very great that all direct 

 evidence of the original nature of the rock has been obliterated. 



1. BocTcs little altered. — My best specimens of comparatively 

 unaltered material are from Langdale ; and I would especially de- 

 scribe one of somewhat coarse grains, containing fragments varying 

 up to y\j inch in diameter. This, to a great extent, is composed of 

 what may be called very vesicular lava or imperfect pumice, the 

 basis of which varies from perfect glass to a completely devitrified 

 felsite, and often shows a perfect fluidal structure. This glassy 

 material is transparent and of pale green colour, and still remains 

 so when partially devitrified by the formation here and there of 

 material which depolarizes light, but yet shows no definite crystals. 

 The next stage is the formation of small dark grains of magnetite 

 or ilmenite, and well-developed felspar microliths ; and the final 

 stage of devitrification consists in the passage of the remaining 

 glass into granular matter, so as to give rise to a sort of felsite. 

 Scattered through the glassy or stony basis are larger crystals of 

 felspar ; and the deposit contains many detached more or less 

 broken crystals of the same, which often enclose portions of the 



