1889.] Mineralogy and Petrography. 907 
consequence of their mode of formation. Its absence from all effusives 
is the result of the comparatively small quantity of oxygen in the air of 
early geological ages. The absence of leucite from intrusive rocks is 
thought to be due to the fact that the mineral can separate from a 
magma only under low pressure. The great stability of el^eolite and 
orthoclase as compared with nepheline and sanidine is supposed to be 
the consequence of their formation under great pressure. So many 
other conclusions of equal interest are indicated throughout the paper, 
that it will well repay reading by any one interested in chemical geo- 
logy or in the study of altered phases of rock masses. The eruptive 
rocks' west and south of Sarn, Caernarvonshire, Wales, comprise gran- 
ites, gabbros, diorites and gneissic diorites, diabase, picrite and dolerite. 
The gabbros and diorites are intimately related, the gneissic varieties of 
the latter having been derived from the former by pressure alteration. 
In the diabases augite is found in idiomorphic crystals and in ophitic 
plates. The hornblende occurs, (i), as an original constituent, enclosing 
grains of augite ; (2), as a secondary product surrounding augite cores ; 
(3), as a zonal growth around augiteswith a corresponding orientation; 
and (4), as a secondary fringe around original hornblende crystals. 
Harker* describes froniMynydd Mawr, three miles west of Snowdon, 
a bluish-gray compact rock, with porphyritic black crystals, without 
idiomorphic outlines, and twinned crystals of feldspar. The groundmass 
of the rock is a fine grained mixture of quartz and oithoclase, in which 
are the above mentioned phenocrysts and small acicular colorless crystals, 
with a faint blue tint when their long axes are parallel to the vibration 
planes of the nicols. They have a high index of refraction and nearly 
parallel extinctions, and are arrayed in flow lines. The large black 
crystals are pleochroic in blue tints, and upon close examination are 
found to be riebeckite, to which species the small acicular crystals are 
thought also to belong. Mr. Diller^hasseparated the mineral supposed 
to be anatase in the periodotite from Elliott Co., Ky., and has had it 
analysed. Its composition is found to correspond with that of perofskite. 
Mineralogical News. — Prof. Clarke « objects to Tschermak's 
theory with respect to the composition of the micas, in that, of the 
four fundamental molecules assumed by this mineralogist as the basis of 
his theory, three of them are unknown in nature, and two are chemi- 
' Marker, Quart. Jour. Geol. Soc, 1888, p. 442- 
