Sullivan — On Augite and Hornblende. 
39 
the pure salt, even when the crystals contain more than half then- 
weight of cupric sulphate. When the limits within which the change 
of composition consistent with the maintenance of any given crystalline 
series are approached, the physical condition of the crystalline mass 
determines in which of the limiting series it shall crystallize. 
The case here supposed is exactly that of the hornblende and augite 
groups. Both these groups constitute a polymeric series of condensed 
meta-silicates, which may be represented by the same general formula 
(M"Si03)'*, where M" represents the dyad metals, magnesium, iron, man- 
ganese, and calcium. When the magnesium predominates — that is, 
when it is to the calcium in the ratios of about 3 : 1 or 5 : 2 — the salts 
crystallize as hornblende ; when the ratio of the calcium and magnesium 
is about 1:1, the salts crystallize as augite: manganese, being, as I 
have above stated, intermediate between the magnesian and calcian 
series, may predominate in crystals of either form — that is, it is strictly 
dimorphic, the Cummington manganese spar probably representing the 
crystalline series of hornblende, and rhodonite the augite series. 
Where the ratio of the magnesium and calcium lies between the two 
extremes — the hornblendic composition and the augitic — the crystalline 
form assumed depends upon the physical conditions under which the 
crystals are formed. We accordingly find in nature many examples 
of the simultaneous formation of hornblende and augite, which, at least 
in those cases which I have been able to analyze the minerals, approach 
very closely in composition. Thus, small black crystals of augite, of 
the form oop. c3opoo.(oopoo).p. have been found on, and partially 
enclosed in, black, elongated, somewhat rounded hornblende crystals of 
the form qop.(oopqo) op. p. from the black basaltic tufa of Czerlochin, 
in Eohemia.^' Herr Hasenkamp mentionsf the occurrence of similar 
crystals at the Pferdekopf, in the Ehongebirge. I also found, many 
years ago, in the latter locality two or three hornblende crystals, with 
small crystals of augite projecting from the faces. The two kinds of 
crystals were nearly of the same composition. The converse of this 
phenomenon has also been frequently noticed. Thus augite crystals 
from Arendal are sometimes found with a number of hornblende crys- 
tals attached to their sides or impressed into them, and in some cases 
so penetrating the mass of the augite that the structure of the latter 
almost disappears. The beautiful green augite of Lake Baikal, in 
Siberia, known as baikalite, is sometimes permeated by white horn- 
blende. Herr Sandberger has also described a case of a lustrous 
hornblende crystal projecting from a well-defined, dull augite crystal, 
from the basalt of Hartlingen. Augite crystals, with projecting horn- 
blende ones, are common enough from that locality. 
But by far the most interesting examples of the simultaneous for- 
mation of forms of the two series is the occurrence of augite crystals 
with a nucleus of hornblende, and of hornblende with a nucleus of 
* Verhand d. Wiirzburg. phys. Gesellschaftg, ix., 32. 
t Blum, Die Pseudomorposen der Mineraheichs, 45. 
