Lower Silurian Lavas of Eycott Hill. By J.C. Ward. 245 
as diabases, and it becomes a question whether the term diabase 
may not thus be best employed to denote a rock belonging to the 
doleritic or basaltic class, in which the doleritie characters are much 
obscured by the presence of chlorite, or its more unstable ally 
delessite or chloropheite, changing to a brownish colour on ex- 
posure. Zirkel remarks, under the head of diabase,* that the 
diffused greenish mineral seems to be chlorite, and probably a 
decomposition product of augite. 
With regard to the accidental minerals, as they may be called, 
of these ancient lavas, it should be remarked that excessive altera- 
tion carried on through such exons of time could scarcely leave 
them untouched, when the prevailing constituents have undergone 
so much change; nevertheless both olivine and apatite may be 
occasionally observed, and there are few characters common to 
modern basalt which may not be recognized either in full or in 
part among some of these ancient Lower Silurian lava-flows. It 
now remains to be seen how far chemical analysis supports the 
idea of the basaltic character of some of these rocks. 
3. Chemical Examination. 
Having previously had several analyses made of the lavas 
occurring in the Keswick district, and found their composition to 
agree with those of a generally intermediate group between the 
basic and acidic series, I became anxious to test some of these more 
northern lavas in the same way, and especially some of those which 
both lithologically and microscopically appeared more truly basaltic 
in character. I therefore selected three samples from the series 
described in this paper, of varying texture and appearance, and 
Mr. John Hughes, F.C.S., has made me the careful analyses which 
will be found in the table on next page under the third, fourth, 
and fifth columns. No. 3 analysis is that of the lava described as 
(12) in our series, and is microporphyritic in character. No. 4 is 
a very compact variety, and described as (13) in our section, and 
No. 5 answers to (15), and igs more like a modern basalt both 
lithologically, and in some respects microscopically, than any 
others. 
A comparison of these analyses with a mean analysis of rocks 
of the doleritic class shows clearly a general correspondence, so that 
chemically there can be no reason against considering some at any 
rate of these old lavas as representing ancient basalts. 
On comparing these three analyses with two of lava from the 
Keswick neighbourhood, it is seen that the latter are markedly less 
basic. In my previous papers I have stated my belief that many of 
the old Cumberland lavas represent the intermediate group answer- 
* *Mikroskopiche Beschaftenheit,’ p. 407. 
