Sottas— On Pitchstone and Andesite. 9] 
composition of the rock; but it may not be uninstructive to express 
its results by molecular formulz as follows :— 
Morecunar Compostrion. 
Per cent. 
Hedenbergite, 7°5 (Si0,), CaO FeO, : : ‘ 18 
See (Si0,), Al,O; K,0 
Sanidine, Or,Ab,, 8 (Si0,), A1,0, Na,0 
Water pyroxene (S102)40 (Al,03), eae oa 5 34 
Quartz, ‘ ay 
Magnetite (not eopaited for), ; : - : = 
101 
The glass probably consists chiefly of the sanidine and what 
we may term water pyroxene molecules. 
Had opportunity been afforded it for complete dehydration a 
considerable quantity of sanidine and quartz in excess of that now 
present as phenocrysts would have probably crystallized out, and 
the hypothetical water pyroxene molecules would have broken up, 
affording free silica, water, and ferro-aluminous pyroxene, which 
would crystallize together with the hedenbergite molecules as 
augite. 
Dr. Haughton mentions the occurrence of another variety of 
pitchstone, presenting an “ oolitic ”’ appearance, in association with 
that just described. This is probably spherulitic, but specimens of 
it are so friable that I haye been unable to prepare thin slices. 
The Museum Collection contains specimens of another glassy 
rock, labelled ‘“‘ Barnesmore, from a dyke 9 feet wide in granite.” 
This proves to be a typical augite andesite, with a structure as 
much hyalopilitic as intersertal. The felspar occurs in long rect- 
angular longitudinal and square transverse sections, frequently 
twinned once or twice, and containing a core of globulitic glass. 
The extinction angle ranges from 11° to 30°. Lither several 
varieties of felspar are present, possibly a series extending from 
oligoclase to labradorite, or more probably the extinctions are 
obtained in various directions in the zone [001,010]. The 
pyroxene is a pale green monoclinic variety, with an extinction 
angle of 40°. It occurs in small, irregular grains, and long, 
fibrous jointed prisms, approximately square in transverse section 
(fig. 2). It is frequently penetrated by the felspar, and then 
45 
