540 W. M. Hutchings — On some Lake-District Bocks. 



Eocks closely similar to the above, macroscopically and micro- 

 scopically, are exposed on both sides of Easedale Tarn, Grasmere. 

 One specimen, from the right side of the Tarn, is very much altered 

 and impregnated with calcite and other secondary matter, reducing 

 its silica-percentage to 52-45. Its ground-mass appears to have been 

 wholly vitreous, and porphyritic felspars, etc., are very scarce. 

 Specimens from the. opposite side of the Tarn are again full of 

 minute felspar-needles, are less altered (free from calcite), and have 

 a silica-percentage of 60'75 ; but here also the poi'phyritic con- 

 stituents are much less abundant than at Harter Fell, etc. 



From these vitreous and minutely " hyalopilitic " varieties, we 

 may see all gradations of development of crystals in the ground- 

 mass, up to the more usual, much coarser- grained andesites, as we 

 mainly have them in Mr. Ward's descriptions. 



An interesting example is exposed in a small disused quarry by 

 the side of the road from SeatoUer to Seathwaite. The rock is grey 

 tinged with green, showing dark spots of chlorite, but no felspars 

 to eye or lens. It is very fissile, splitting into tolerably thin plates, 

 and approaches more nearly to a slaty cleavage than any other lava 

 known to me in the district ; though I am told there are cases of 

 lavas which are sufficiently cleaved to be worked for roofing-slates. 

 The ground-mass-felspars in sections from this quarry are still 

 small, but larger than in the rocks just described, ranging up to 

 -g-g-o of an inch as maximum. Porphyritic felspars are greatly 

 altered, being almost wholly replaced by calcite, and deformed by 

 pressure and shearing; but the pseudomorphs after augite are mostly 

 but little damaged, and show more numerous recognizable forms 

 than in any other slides I have. The ground-mass is completely 

 permeated by infiltrated calcite, of which there is so much present 

 altogether that the silica-percentage of the rock now only reaches 

 61'6, though originally it must have been fully as high as in the 

 average local andesites, if not higher. 



The numerous lavas on Dale Head, High Scawdell, and Seatoller 

 Fell, present an interesting variety, as we find here not only the 

 more normal Lake District andesites, but also types which seem to 

 lie between these and the basic rocks, and to require such terms 

 as doleritic (better basaltic) andesite, or andesitic dolerite (better 

 basalt), to describe them ; — that is, if we adhere to the more usual 

 custom of distinguishing between andesites, and dolerites or basalts, 

 as extremes of a series. Of course they all pass imperceptibly into 

 one another, and there are petrologists who call the whole series 

 andesites, as does Professor Cole in his system of classification 

 ("Aids in Practical Geology"), simply distinguishing them at the 

 ends of the line as " trachytic andesites " and " basaltic andesites." 

 Eock-classification, in the present state of the subject, is very much 



length of these as XT^th of an inch. They are, therefore, very much larger than 

 in the rocks referred to by me, and from his description the Lodore lava was evidently 

 very much less vitreous than these. It resembles, as Mr. Teall points out, the 

 ground-mass of the average andesites of the district ; a ground-mass in which the 

 felspars are above the average sizes of typical andesites elsewhere. 



