ASSOCIATED METAMORPHIC ROCKS OE THE LAKE-DISTRICT. 591 



I. Lithological Character. 



1. General relations. — I have already alluded in a previous 

 paper * to the extreme alteration which many of the beds of vol- 

 canic ash have undergone, and given my reasons in some detail 

 for considering very trap-like beds to be merely altered strata 

 of originally fragmentary origin f . It is therefore only necessary 

 to state here that a perfectly gradual passage may be traced from 

 beds of volcanic ash in which every separate fragment is clearly dis- 

 cernible, through others in which the original ashy structure is only 

 revealed by weathering, to rocks, immediately in the proximity of 

 the granite, of a close trap-like texture, with imperfect felspar 

 crystals frequently developed porphyritically, fully charged with 

 finely disseminated mica, and occasionally showing free quartz 

 clearly visible to the naked eye, outlying patches of a fine-grained 

 granite being in some cases met with at a little distance from the 

 main mass. 



That there are occasional sudden transitions from a less altered 

 to a much more highly altered rock, is only what might be expected 

 when it is considered that we have a series of beds of all degrees of 

 coarseness and fineness, and very probably differing somewhat from 

 each other in their chemical composition. It may readily be imagined 

 that a fine volcanic dust, when much metamorphosed, would present 

 a somewhat different appearance from a coarse ash or breccia altered 

 under similar conditions : one kind of bed may yield itself more 

 readily to metamorphism than another; hence, while in some 

 cases the junction of these altered rocks with the granite may be 

 sharp and definite, in others a much more gradual passage is 

 observable. The general order of change met with on approaching 

 these granitic masses, and more especially that of Eskdale, is the 

 following, subject of course to the causes of variation just men- 

 tioned. 



2. First Stage. — At a distance, frequently of several miles, from 

 the nearest granite, the outlines of the fragments forming a bed of 

 ash or breccia are hazy and indistinct, except on the weathered sur- 

 faces. Also the finer material between the larger fragments seems 

 often to have acquired a kind of flow around them, giving rise to a 

 streaky-looking rock, the streaks, however, being discontinuous and 

 quite unlike the comparatively regular and continuous though con- 

 torted lines observable in some undoubted felstone-lavas, such as 

 those of Wales and that in connexion with the Coniston Limestone 

 west of Shap. If the ash be wholly fine-grained, a nearer approach 

 is made to the appearance of viscous flow j)resented by many highly 

 silicated lavas ; but even in this case remains of original lines of 

 bedding, the passage laterally into a less altered- looking rock, and 



* " The Microscopic Eock- Structure of some Ancient and Modern Volcanic 

 Rocks," Quart. Journ. Geol. Soc. vol. xxxi. p. 388. 



t See also the Survey Memoir on the G-eology of the Northern part of the 

 Lake-district, by the author. 



Q.J.G.S. No. 124. 2r 



