542 W. M. Hutchings — On some Lake-District Rocks. 



A rock from Seatoller Fell is of some interest. It is a very com- 

 pact, dark rock, quite free from vesicles. Its numerous porphyritic 

 felspars are all much altered, though still perfect in outline and 

 relationship to the ground-mass. There are no definitely recognizable 

 larger pseudomorphs of chlorite after augite, though good large 

 patches of chlorite are present. The ground-mass consists of perfectly 

 fresh felspar-laths of a rather larger size than usual, all twinned, 

 and showing by many extinction measurements that labradorite is 

 dominant. These felspars lie in among a yellow-brown, dimly- 

 polarizing, partially devitrified glassy base, full of minute dark 

 dusty matter. The amount of this base is considerable. In among 

 the felspars and glass are a large number of good-sized irregular 

 grains of very pale-green hornblende, which is clearly secondary 

 after former augite. In many cases the felspars penetrate the horn- 

 blende in a manner showing that a certain amount of ophitic 

 structure was present, but more usually the grains lie wedged in 

 the angles formed by the felspars. There are also numerous bits 

 of chlorite occurring in exactly the same manner as the hornblende. 

 Originally the rock must have contained a considerable amount of 

 augite in this form, but none of it is now seen unaltered. There is 

 abundance of finely disseminated chlorite and grains of epidote, 

 some of which appears to represent a smaller generation of augite in 

 the ground-mass. The sp. g. is 2-88, and the silica-percentage 

 53-55. This rock would have to be classed as " andesitic basalt." 



Hornblende secondary after augite is not at all frequently observed 

 in these rocks. There is another lava on Seatoller Fell, a normal 

 andesite, in which it occurs as prismatic grains of deep green colour, 

 this being the only case in which I have observed hornblende in 

 one of the regular local andesites. Original hornblende is never 

 seen nor any sign that it ever was present. It seems probable that 

 hornblende-andesites were never represented in the district. 



Enstatite has been shown by Prof. Bonney (GtEOL. Mag. 1885, 

 p. 77) to have been a constituent of some of the Eycott Hill lavas ; 

 and Mr. Teall supposes it probable that the andesites of the district 

 contained it. I have sought carefully for it in many rocks from 

 various localities, but have not found it, nor any secondary product 

 which suggests it, in any undoubted lava. In a rock from Dale 

 Head, of which I am uncertain whether it was a flow or an ash, 

 there are good-sized serpentine-pseudomorphs after some mineral, 

 which I think was very probably enstatite. The secondary naineral 

 is not bastite. Again, in an ash or tuff from the summit of Sargeant 

 Man a good deal of serpentine appears, some of it in the form of 

 small prismatic pseudomorphs, contained in andesitic lapilli, strongly 

 suggestive of original crystals of enstatite ; and in a tuff from 

 Glaramara one good-sized serpentine-pseudomorph is also seen, the 

 form being quite that of the enstatites of andesites, etc. 



It would seem likely that enstatite was a rare constituent of 

 these rocks. 



A rock occurs at Thornthwaite Crag, a little below the tall Cairn, 

 which differs sufficiently from the usual andesites to be worthy of 



