690 WHITMAN CROSS 
The monchiquite of Shelburne Point agrees very closely in 
composition with this analcite-basalt. Its high alumina would 
no doubt be materially reduced if TiO, and P,O, were deducted. 
Of this particular rock Kemp quotes Professor Rosenbusch, to 
whom it had been shown, as saying that it is a ‘‘true monchiquite 
yy 
of typical habit. From a microscopical examination of a speci- 
men of this rock which is in the reference collection of the 
Geological Survey it is plain that there is a great difference in 
the development of the two rocks. In the Shelburne Point rock 
there is a cloudy gray base of indistinct radially fibrous struc- 
ture and of weak double refraction. It is neither glass nor nor- 
mal analcite at present, whatever it may have been originally. 
It is clear from the analysis, however, that the rock of Shelburne 
Point and that of The Basin resulted from magmas of almost 
identical composition. 
The original monchiquite (analysis VI) varies somewhat from 
the others, being higher in alumina, iron and alkalis and lower 
in lime and manganese. Its alumina must contain considerable 
phosphoric acid. Its isotropic base, interpreted by Hunter and 
Rosenbusch as glass, has been shown by Pirsson’s recalculation 
of Hunter’s analysis to have practically the composition of 
analcite. 
The plagioclase-basalt of Saddle Mountain is so near to 
the monchiquite of analysis VI in chemical composition that 
if its magma had contained 2 per cent. more water it might 
in all probability have yielded a monchiquite. As many normal 
basaltic dikes occur near The Basin it seems reasonable to 
assume that the analcite-basalt magma contained more water at 
eruption than did those of the plagioclase-basalt dikes. 
In the original description of monchiquite Messrs. Hunter and 
Rosenbusch call attention to the association of all the rocks of 
that type then known to them with eleolite-syenite, and express 
the belief that magmas of this kind must have given rise to the 
monchiquites together with some complementary acid rocks. 
The expression of this belief in the positive form of the title to 
Sell, WOv/5 Uy > (Ga Sey 19 B50 
