90 Scientific Proceedings, Royal Dublin Society. 
Tait ; in Column III. also of an Arran pitchstone, by J. H. Player, 
taken from Teall’s “ British Petrography,” p. 347. 
I. IT. III. 
Silica, 7 : 64:04 66°03 72°6 
Alumina, ‘ : 10°40 12°55 12°4 
Ferric oxide, : 9°36 2°75 “7 (FeO, 1°1) 
Lime, we 4-24 2°80 9 
Magnesia, . : none 2°33 trace 
Potash, ‘ ‘ 3°63 4:13 4:7 
Soda, . A ; 2°91 5°02 1-7 
Loss by ignition, . 5°18 4°30 5:2 
99°71 99°81 99°3 
Sp. gr., : 2°41 — 2°34 
The absence of magnesia from a rock containing so much pyr- 
oxene is remarkable, but not unique; it would seem to indicate an 
aluminous hedenbergite as the chief pyroxenic molecule, a sugges- 
tion supported by the high extinction angles observed under the 
microscope. 
From the analysis, Dr. Haughton calculated a possible mineral 
composition for the rock as follows :— 
Quartz, ; : j 7°33 
Felspar, : : . 62°55 
Stilbite, : : : 29°83 
99°71 
It is interesting as illustrating the aid afforded by the micro- 
scope in checking calculations from analysis, to point out that in 
this case, where the mathematical computation takes no account of 
paste, the microscope proves it to be present in the state of glass as 
the chief constituent, while in granite, where the equations leave a 
surplus of material as paste, the microscope shows that paste is 
altogether absent. 
The bulk analysis is insufficient for determining the mineral 
