W. M. Hutchings — On some Lake- Didrict Rocks. 639 



tinguishable. There appears to have been a good deal of interstitial 

 glass originally, but it is now devitrified, and obscured by alteration. 

 The sp. g. is 2-82 and the silica-percentage 51-35. This rock, 

 then, corresponds to a porphyritic augite-dolerite, or probably, more 

 correctly, basalt. 



Turning to the andesites, there is a type of them abundantly repre- 

 sented at one part of the district, and apparently recurring at several 

 other points, which is very interesting and differs in several ways 

 from the usual varieties as described by Mr. Ward, or as summarized 

 by Sir A. Geikie (Presidential Address to Geol. Soc. 1891). It 

 consists of rocks of mostly a grey-green or grej^-blue colour, — a 

 colour not easy to exactly describe, — with resinous lustre and 

 extremely splintery fracture. On a newly-broken surface small 

 dark spots are seen, but hardly ever any sign of a felspar, and 

 when seen these are very small. These particular rocks are exposed 

 largely at Harter Fell, Mardale, making the main part of the cliff 

 facing towards Haweswater ; on the ascent from Nan Bield Pass on 

 to High Street ; and in cliffs on the right side of Kentmere Valley, 

 some distance below the reservoir. 



A series of sections prepared from specimens taken at all three of 

 the above localities shows that the rock is essentially the same in 

 nature at all of them. There are numerous felspar-crystals, sharp 

 in outline, though inwardly much altered, porphyritic in a ground- 

 mass which originally varied from a wholly glassy base to an 

 intimate mixture of glass and exceedingly minute felspar-microlites. 

 There are chlorite-pseudomorphs after augite, distinct in form, as 

 well as indefinite patches and streaks of chlorite. The entire ground- 

 mass is permeated by chlorite in minute flakelets, with small granules 

 of epidote, calcite, and other matter. 



Sections from the summit of Harter Fell, close to the double cairn, 

 show a good deal of perfectly fresh augite as well as many pseudo- 

 morphs. The ground-mass here consists nearly wholly of devitrified 

 glass; a dimly-polarizing, speckly, felsitic mass with scarcely any 

 felspar microlites at all. Sections from Kentmere, again, have a 

 ground-mass of the most typical " hyalopilitic " nature, wherein the 

 same devitrified glass is quite full of tiny laths and needles of felspar, 

 showing well-developed flow-structure round the porphyritic felspar- 

 crystals. The lai'gest and most clearly-developed of these felspar- 

 needles are close around —oVoth inch in length and extinguish quite 

 parallel. The average size is very much less. 



There is no doubt that, though varying more or less in detail from 

 place to place, and even within very short distances, we have here 

 an altered augite-andesite of a much more vitreous nature than the 

 dominant type of lavas of the Lake District. A charactei'istic 

 specimen from above Nan Bield has the sp. g. 2-65 and a silica- 

 percentage of 57"55.^ 



' Mr. "Ward refers to rocks of a ^rey-blue colour, very compact, -which are 

 outwardly not unlike those above described. The closest resemblance is in a lava 

 from near Lodore Hotel, which is singled out by him on account of its non- 

 porphyritic nature and the smallness of its felspar-needles. He gives the average 



