Lower Silurian Lavas of Eycott Hill. By J.C. Ward. 241 
deal of quartz in cavities. There seems to be scarce any unaltered 
augite, but, to judge from analogy, much of the chlorite must re- 
present that mineral. Some of the green mineral has a trans- 
versely fibrous structure when seen with crossed prisms. 
(8) Skiddaw Slate Band. 
(4) Lava (porphyritic) —This highly interesting and beautiful 
bed is about 100 feet in thickness, and in lithological structure 
shows a compact greenish-blue base containing dark-green spots of 
a soft mineral, and large porphyritically imbedded felspar crystals, 
many of them an inch long. 
Its microscopic character I have already described in the 
‘Keswick Memoir’ (p. 20), and in my paper on “The Microscopic 
Structure of some Ancient and Modern Volcanic Rocks.” * In 
both cases coloured drawings are given. That in the ‘Survey 
Memoir,’ fig. 10, plate ii, shows a fine augite twin imbedded in 
the base of acicular felspar prisms, the spaces between which are 
filled up with a dirty green and brown pseudomorphic mineral 
(chlorite and chlorophzite) and numerous crystals of magnetite. 
The figure in the Quarterly Journal (fig. 6, pl. xvii.) shows a part 
of the same more highly magnified, and that in the Plate illustrating 
this paper (Fig. 1) shows the character of the crystalline base, one 
of the green pseudomorphs, and a portion of one of the large 
plagioclase crystals. These last are much cracked, and contain 
glass cavities, portions of the base, and grains of magnetite; they 
present very finely the banded structure peculiar to this group of 
felspars when viewed with polarized light. 
Augite, in crystals and grains, occurs pretty plentifully; much is 
in the form of pseudomorphs however (the soft dark spots before 
mentioned). The large twin spoken of above as figured else- 
where, contains an interesting example of a glass cavity with a 
bubble and two magnetite grains, with the following dimensions : 
Glass cavity +525, of an inch in diameter. 
Bubble =, of an inch in diameter. 
Tao i 21 1 1 i 
Magnetite grains the 3,5 and ~53¢5 Of au inch. 
Some of the green pseudomorphs seem to be after olivine, pre- 
senting the form and much-fissured appearance of that mineral. I 
have detected grains of olivine in an unaltered condition in some of 
these lavas, and therefore I think there can be no doubt that both 
it and augite were common constituents at one time, though both 
have been so much replaced by pseudomorphic minerals through 
subsequent alteration. The top of this lava is beautifully vesicular 
in parts, the vesicles being drawn out along the line of flow, and 
filled with chlorite, chalcedony, and calcite. 
* “Quart. Journ. Geol. Soc.’ vol. xxxi. p. 406. 
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