78 Prof. T. G. Bouncy — The Endatitic Lavas of Ei/cott Hill. 



and greener parts mentioned above do not extinguish simultaneously. 

 The mineral contains a few scales of iron-glance and some light 

 brown granular endomorphs, which occur also in the ground-mass ' 

 but is generally rather free from inclusions. Obviously we have 

 here a magnesian silicate containing some iron, which is more or less 

 converted into a kind of serpentine. Clearly it is not olivine, neither 

 is it normal bastite. It does not correspond with hj'persthene, 

 especially with the small crystals of that mineral which have of late 

 been noticed in so many andesites. Its general aspect agrees with 

 that of some of the more or less altered enstatites and bronzites 

 with which I am familiar from my studies of serpentines, and it 

 reminds me also of the representations given of altered enstatite and 

 of so-called bastite by Fouque and Levy (Mineralgie Mtcroscop. Roches 

 Ervptives Francnises, plates xxvii., liii., and liv.) 



The ground-mass, in which the above-described minerals are 

 embedded, consists of lath-like crystallites of plagioclase felspar, of 

 grains and imperfectly developed little crj'stals of augite, and of 

 crystals and granules of iion-oxide, which probably is mostly hematite. 

 There are occasional scales of iron-glance. These are thickly set 

 in a bi'own glass-like base. This when examined with objectives of 

 fairly high power — say from J to ^ of an inch — becomes paler and 

 greyer in colour, and is found to be crowded with dark granules and 

 blackish belonites, which are sometimes slightly curved. These 

 occasionally seem to interlace so as to form a kind of network. 

 Similar microliths, but with a brush-like grouping, are figured by 

 Eosenbusch in a hydrotachylite ^ [Microscop. Phyaiograph. vol. i. 

 plate iii.). These belonites occur in the slide prepared from Mr. 

 Postlethwaite's specimen ; in that from a specimen collected by my- 

 self they are, if not absent, exceedingly rare. The general character, 

 however, of the ground-mass of the latter corresponds with that of 

 the former. So far as I can ascertain, the base is still a true glass, 

 and has not undergone devitrification. 



After I had informed Mr. Postlethwaite of the result of my 

 examination of his first specimen, he again visited Eycott Hill, for- 

 warding tome fresh fragments of the redder variety broken from the 

 rock in situ, and then a block, also obtained in situ, which exhibited 

 a passage from that variety to the normal rock. I have had a slide 

 cut from the extreme parts of this, for the transition from the one 

 tint to the other is too gradual to offer anything like a junction. 

 The two slides when examined under the microscope are almost 

 identical, the only difference being that in the redder variety small 

 scales of burnt-sienna coloured iron-glance are more numerous, and 

 in its ground-mass the dark belonites are more frequent than in the 

 normal rock. Also I think that the base of the former, when viewed 

 with a low power, is a shade browner than that of the latter. The 

 description given above serves as a whole for these slides also. Each, 



1 They are not isotropic. Probably they are alteration products, possibly ferruginous. 

 They often occur in somewhat altered igneous rocks. 



^ Similar needles are figured by Prof. Judd and Mr. Cole in their admirable paper 

 on the Basalt Glass of the Western Isles of Scotland, Q.J.G.S. vol. xxxix. p. 444. 



