ro asia 
XXIX. 
A CONTRIBUTION TO THE THEORY OF THE ORDER OF 
CRYSTALLIZATION OF MINERALS IN IGNEOUS ROCKS. 
By J. A. CUNNINGHAM, A.R.C.Sc.1., B.A. 
(Prares XXII. ann XXIIT.) 
[COMMUNICATED BY PROFESSOR GRENVILLE A. J. COLE, M.R.I.A., F.G.S. | 
{ Read Junz 20 ; Received for Publication Noy. 23, 1900; Published May 7, 1901.] 
I micur fairly be expected to offer some apology for bringing” 
this Paper before the Royal Dublin Society, since it is only so 
recently as March 21st of the present year that the subject was 
discussed and a theory put forward by no less distinguished an 
authority than Dr. John Joly, F.R.S.1. But after considering the 
matter carefully, L cannot help thinking that the experiments on 
which Dr. Joly’s theory was based are capable of a very different 
interpretation. In the present Paper I have therefore attempted 
to reinterpret them, and to correlate them with other experi- 
mental data, and to show how they point towards another theory 
which was formulated by Bunsen’ in 1850. 
The facts which demand a theory in order to reconcile their 
apparent inconsistencies can be briefly stated. By the study of 
thin sections of rocks petrologists have been able to determine 
with comparative ease and certainty the order in which their con- 
stituent minerals have consolidated. This order can, in general, 
be expresed in terms of Rosenbusch’s® Law as being the inverse of 
the order of basicity of the minerals concerned. Thus, in parti- 
cular, quartz is always last to crystallize in deep-seated plutonic 
rocks. ‘his order was at once seen to be almost exactly the reverse 
of what one would expect from the relative fusibilities of the rock- 
iJ. Joly, ‘‘Theory of the Order of Formation of Silicates in Igneous Rocks,’’ 
Proc. Roy. Dublin Soc., vol. ix. (N.S.), pt. ii. (1906), p. 298. 
2R. Bunsen, ‘*‘ Ueber den Hinfluss des Drucks auf die chemische Natur der 
plutonischen Gesteine,’’ Pogg. Ann. Phys. u. Chem. 81 (1850), p. 562. 
3 Rosenbusch, N. Jahrb. Min. Geol. u. Pal. (1882), Bd. vi., p. 1. 
SCIENT. PROC. R.D.S., VOL. IX., PART IV. 2H 
