~ 
MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 651 
amount of iron oxide which corresponds to 1°49 per cent. of MgO, 
and assuming that the remaining iron is wholly associated with 
TiO, to form the magnetic ore, we have the cemposition of this ore 
as follows :— Mol. Rel. 
TIO) 0. 245k 83:80. 4162 
Fe,0, .. 2470 3356 2101 
FeO «rnc PES9y 8a4ee held 
73°60 100-00 
Another sample from the same rock was partially analyzed. 
*6150 gramme gave °1635 gramme TiO,, and -3639 gramme Fe,0, 
(total iron). If we assume that the total Fe is associated with Ti0, 
to form ilmenite (FeO Ti0,+xFe,O,), then we have as the composi- 
tion of the mineral— 
ORE AML MOLES 31-99 
Meer ee Soy ere! 28-72 
Bee ees ree tem 39-29 
100-00 
These results can of course only be regarded as rough approxi- 
mations to the truth. In the second case no determination of FeO 
was made. In the first case this was done, and the result shows 
that there is more FeO than is required to form the compound 
FeOTi0,—that is, more than is required for true ilmenite. 
The analysis and also the physical properties of this substance 
agree with the assumption that we are here dealing with a mixture 
of magnetite and ilmenite, and the question arises—Is there any 
evidence that these two minerals occur intergrown together? Such 
evidence is supplied by the work of Renard and Dela Vallée Poussin*, 
Neeff, and Kicht. Neef found, on treating a certain section of © 
diabase with hot hydrochloric acid, that the opaque iren-ore was 
only partially destroyed; a fine network, due to two parallel series 
of lamellz of ilmenite intergrown so that the angles between the 
lamelle corresponded to the angles of the fundamental rhombo- 
hedron of ilmenite, remaining behind. The mineral removed was no 
doubt magnetite, as this substance is much more readily attacked 
by hot hydrochloric acid than ilmenite. Kiich obtained precisely 
similar results in the case of a rock from West Africa. Renard and 
De la Vallée Poussin figure such a network as that referred to in 
plate i. fig. 6, of the work cited below. Lustly, the alteration of 
the Whin-Sill substance into leucoxene takes place along two sets of 
parallel planes (see Pl. X XIX. figs. 4, 5, and 6). We may conclude, 
therefore, that we are here dealing not with one definite mineral, 
but with an intergrowth of two, viz. magnetite and ilmenite. 
It is not a little interesting to observe that we have in this rock 
three separate instances of the intergrowth of two minerals, viz. 
* «Mémoire sur les roches dites plutoniennes de la Belgique et de I’ Ardenne 
francaise.” Mémoires couronnés de Académie Royale de Belgique. Tome x1. 
H50. 
id t “‘Ueber seltenere krystallinische Diluvium-Geschiebe der Marke,” Z. D. G. G. 
xxxiy. p. 470. 
. { “Beitrag zur Petrographie des west-africanischen Schiefergebirges.” Min. 
Mitth. Neue Folge, vi. p. 129. : 
