650 J. J. H. TEALL ON THE CHEMICAL AND 
the definite structure cannot be recognized, but in which there is 
every reason to believe that it actually exists. Is it possible that 
the mineral now under consideration is a submicroscopie association 
of monoclinic and rhombie pyroxene? The idea here thrown out 
was suggested to me in conversation by Prof. Rosenbusch. 
Returning once more to the general consideration of the pyroxene, 
we observe that, so far as the ratios Ca: Mg: Fe are concerned, it is 
intimately connected with the pyroxene of the Connecticut diabase 
and with that of a basaitic rock from the Faroe Islands. Both these 
pyroxenes show the same tendency to develop a laminated struc- 
ture parallel to oP. To what extent this tendency is connected 
with the composition I am, of course, unable to state. Two other 
points require attention. The ratio of R: R is greater in the ~ 
Tyne-Head pyroxene than in that of the other two localities, and 
the amount of water is considerable. It is possible that the de- 
velopment of the laminated structure is connected with the addition 
of water. Thus the pyroxene of the Cauldron-Snout rock is less 
conspicuously laminated than that of the rock from Tyne Head, and 
it contains less water. A comparison of the two analyses of the 
Whin-Sill pyroxene shows that the coarse variety is closely allied 
to, though not identical with, the common rock-forming mineral. 
Opaque Iron Oxide.—In the compact varieties, this substance 
occurs in the form of extremely minute specks, evenly scattered 
throughout the section. In the finely erystalline varieties it occurs 
in the form of grains, and im the coarse varieties in the form of ex- 
tremely irregular ragged masses. Many of the sections, however, show 
forms which are supposed to indicate ilmenite (Pl. XXIX. figs. 4,5, 
6, and 7), and the white alteration produced may often be observed. 
All the grains, which consist either wholly or in part of the opaque 
iron-ore, may be extracted from the powder by a weak bar-magnet. 
True ilmenite is not attracted by a strong electro-magnet. Owing 
to the extremely ragged character of this titaniferons ore, the grains 
extracted by the method above referred to are largely eomposed of 
felspar and augite. An attempt was made to purify the substance 
by treatment with hydrofluorie acid, but the result was unsatis- 
factory, as the substance itself was more or less decomposed with 
separation of titanic acid. An analysis of the impure substance 
yielded the following result :— 
RO IS SES US 12-16 
Pit): 2%, ee 24-51 
AO” 2 ieee 3°36 
Pe. Oe* Aes he 24-7 
PEO Te Fee 26°54 
Oa sre Tigh 4-40 
MeO iA yr 1-49 
97-16 
This analysis leaves much to be desired; still it will give some 
idea of the composition of the ore. Since augite occurs as a visible 
impurity, we shall probably not err seriously if we assume that the 
magnesia is wholly present in that mineral. Deducting, then,an 
