656 J. J. H. TEALL ON THE CHEMICAL AND 
The excess of alumina, iron, and water, and a portion of the 
excess of quartz, are no doubt due to the alteration which has 
taken place, kaolin and chlorite both being present. A large portion 
of the silica, however, no doubt represents free original quartz. If 
the felspar molecules were associated with each other to form a 
single mineral, the ratio of Na (K) : Ca would be, according to the 
above, 2°72: 1. But lime has certainly been removed from the 
rock; and if we allow for this, on the basis that the excess of ALO, 
was wholly present in molecules of anorthite, then this would make 
the above ratio 1°58: 1. The discussion of the bulk-analysis leads 
us, therefore, again to the conclusion that the original felspar, if of 
one species, was andesine. 
We cannot construct a table for the Bourgovicus rock, because 
the microscope shows that both bronzite and the colourless pyroxene 
occur, in addition to the common pyroxene. 
Relations of the Whin Sill to other Rocks. 
If we compare the rock of the Whin Sill with the dykes described 
in my former paper (Q. J. G.S. vol. xl. p. 209), we are at once struck 
with its resemblance to those of Hett and High Green. The figure 
which is given of tke rock of the High Green dyke (J. c. pl. xiii. fig. 2) 
represents faithfully the general structure of the Whin Sill. The 
ilmenite of my former paper is, no doubt, the same mineral as the 
titaniferous magnetic iron-ore of the present paper. 
There are some very interesting points both of resemblance and 
difference between the Whin Sill and the eruptive rock of Penmaen- 
mawr, described by Mr. J. A. Phillips (Q. J. G.S. vol. xxxiii. p. 423). 
These two rocks are composed of the same minerals, associated in 
very different proportions. Thus the freshest specimens of the 
Penmaenmawr rock are seen under the microscope to consist of 
bronzite*, monoclinic pyroxene with lamination parallel to oP, plagio- 
clase, titaniferous iron ore, and quartz, both in the form of grains 
and associated with felspar in micro-pegmatite. In the Whin Sill 
the monoclinic pyroxene is largely in excess of the bronzite ; but 
in the Penmaenmawr rock the reverse relation holds—the bronzite 
is very largely in excess. A comparison of the bulk-analyses shows 
that the Penmaenmawr rock possesses a larger proportion of felspar 
than the Whin Sill. 
Comparing our rock with the Carboniferous dolerites of central 
England we are struck by the absence of olivine. I have not seen 
* T think this is the mineral that Mr. Phillips referred to hornblende. I 
have not seen any hornblende in the rock, and at the time that Mr. Phillips's 
paper was written the methods of distinguishing the crystal-systems by refer- 
ence to the extinction-angles was not well known. It is not surprising, there- 
fore, that he should have taken the pleochroic bronzite for hornblende. I 
may state that my attention was specially directed to the Penmaenmawr rock 
by the following sentence in Rosenbusch’s work, ‘Die massige Gesteine,’ 
p. 352 :—“ Die Trappe von Conway und von Penmaenmawr in Wales gehoren 
zu den enstatitfiihrenden Diabasen und zwar zu den typischesten Reprasen- 
tanten derselben.” 
