OBSERVATIONS ON THE STRUCTURE OF ROWLEY RAG. 265 
The analysis of the vein gives very curious and interesting 
results, which are tabulated below: 
■ 
I. 
II. 
Sp. Gr 
. 2-70 
Sp. Gr. 2-58 
Silica. 
48-8 
. 58-3 
Alumina . 
18-1 
. 17-9 
Ferrous oxide ... 
7-2 
. 3-0 
Ferric oxide 
8-5 
. 2-5 
Lime. 
8-4 
. 2*1 
Magnesia . 
4*9 
. 1-9 
Potash . 
1-9 
. 5-9 
Soda. 
3-7 
. 5-2 
Loss at red heat 
3-6 
. 2-7 
100-1 
99*5 
It should be mentioned that in 
these analyses the titanic 
acid is not determined ; the silica and the alumina are there¬ 
fore slightly too high, as the rock contains about two per cent, 
of titanic acid. 
I. Gives the percentage composition of an ordinary speci¬ 
men of the unaltered “ Blue ” stone close to the vein. 
II. That of the vein in question. 
We see, then, that the portions of the melted mass which 
consolidated last contain nine per cent, more silica, and a 
very much increased amount and different proportion of alkalis, 
with, as was to be expected, a lower Sp. Gr. 
In a paper read to the Section in October last, and 
published in the “ Naturalist ” for January, 1885,1 mentioned 
very similar relations as to composition between the main 
mass of the rock of Pemnaenmawr, in North Wales, and 
certain grey or white veins, evidently contemporaneous, 
which occur in it; and the same has been described in the 
case of the great Cockfield Dyke of the North of England, 
with regard to the glassy residual base (see Mr. Teall’s paper 
in the “Quarterly Journal of the Geological Society ” for 
May, 1884). In the number of the “ Geological Magazine” 
for March of this year Mr. Teall again shows that the glassy 
base of the hypersthene andesites of the Cheviots has the 
chemical composition of a quartz felsite, and contains five per 
cent, more silica than the general average of the rock. 
He also throws out the suggestion that the quartz felsites 
of the district may possibly be the more acid parts left after 
the separation of a certain amount of crystals from the 
andesite magma, the separation having been effected either by 
the subsidence of the crystals or by the still fluid residuum being 
squeezed out from among them “as water from a sponge.” 
