Dr. A. Wilmore—Uralite and other Amphiboles. 367 
These two processes cannot, of course, be sharply separated. There 
is every possible gradation and combination of the two. The second 
process may have proceeded to some extent, and then the former may 
be superimposed, with the result that, in some cases, there has clearly 
been a passage from pyroxene through fibrous hornblende to compact 
brown or green hornblende. 
(3) There may be magmatic resorption and corrosion of pyroxenes with the 
result that amphiboles of various kinds, usually aluminous hornblendes, are 
formed on the periphery, and along and near the planes of penetration, and in 
the extreme case there may be complete magmatic reconstructicn of the pyroxene 
into hornblende. When this extreme result has been reached it, will be clearly 
impossible to determine the ‘secondary’ character of the hornblende unless 
there is a series of partially resorbed crystals to act asa guide. It is, of course, 
possible to describe such a hornblende as original. 
This will probably take place in the zone of anamorphism, and in 
the upper part of that zone; while the converse change may be 
looked for in the lower part of that zone, and amphiboles of specific ’ 
gravity 3 to 3:3 may be converted into pyroxenes of specific gravity 
3°2 to 35 approximately. 
Note I, on Melting-points of Pyroxenes and Amphibole—The following results 
have been published by the authors named :— 
Cusack. Doelter. Brun. 
Augite GIR fic 1085° 1230° 
1199° 1200° 
Hornblende 1187° 1065° 1060° 
1200° 1155° 1070° 
Cusack: vide Roy. Jr. Acad., vol. iv, pp. 399-413. 
Doelter : Tscherm. Min. Petr. Mitth., vols. xx-xxii ; Physikalisch-Chemische 
Mineralogie, pp. 99-100. 
Brun: Arch. Sci. phys. et nat., Geneva, vol. xii, pp. 352-75. 
Vogt, Day, and others have obtained much higher melting-points for the simple 
pyroxene diopside, up to 1375°. 
Allen, Wright, and Clement (Amer. Journ. Sci., vol. xxii, pp. 385-438) give 
1521° as the melting-point of the high temperature stable form of the Mg Si 03. 
Instead of simply regarding this simple metasilicate as dimorphous they regard it as 
tetramorphous, as follows :— 
Monoclinic pyroxene ; rhombic pyroxene (enstatite) ; 
Monoclinic amphibole ; and rhombic amphibole. 
The pyroxenes are stable at high temperatures, the monoclinic form being the 
most stable ; the amphiboles are low temperature minerals, and probably change into 
stable pyroxenes at high temperatures. Hence we can understand the formation of 
pyroxene from amphibole as recorded by Lacroix in the lavas of Auvergne. (See 
also Harker, Zhe Natural History of Igneous Rocks, Methuen, 1909, pp. 155-8.) 
Note II, the Angle Relations of Augite and Hornblende.—Take the average prism 
angle of augite as 874° and that of hornblende as 1243°. Now take half of each of 
these angles. The following important relation is now evident :— 
tan 624° is approximately 1:90, 
tan 43#° is approximately -95. 
That is, the tangent of half the prism angle of hornblende is twice the tangent of 
half the prism angle of augite. : 
