166 rT, G. BONNEY AND F, T. §, HOUGHTON ON MICA-TRAPS 
The principal subordinate members of the mica-trap group have 
been named by many petrologists minette and kersantite*, the 
former denoting those where the felspathic constituent is chiefly 
orthoclase f, the latter chiefly plagioclaset. After this subdivision 
great diversity appears in the use of the terms, little regard being 
paid to the crystalline condition of the ground-mass. 
By Prof. Rosenbusch§ the term minette is used as equivalent to 
mica-syenite, and kersantite to mica-diorite, or at least as denoting 
the more conspicuously micaceous varieties of these rocks. The 
group corresponding generally with the latter in chemical composi- 
tion, but differing from it in having a micro- or crypto-crystalline 
ground-mass, is named by him kersantite-porphyrite ; but we do not 
find a term for the corresponding rock in the former group. For 
this, then, we propose the analogous term minette-felsite||. It is 
quite possible that one or two of the rocks which we are describing 
in this paper may once have had even a glassy base; but in their 
present condition we do not feel able to affirm this positively, as the 
structure has been so much obscured by subsequent decomposition 
and micro-mineralogical change. 
The amount of this has frequently been very great, some of the 
original constituents having been replaced wholly, or almost wholly, 
by pseudomorphic or other secondary products. We have thus 
oiten been obliged to speak very doubtfully as to the original nature 
of some constituents. 
A few preliminary remarks on some of the minerals observed in 
these rocks may save time in the subsequent descriptions. 
Mica.—This mineral is abundant in almost all the specimens. It 
is usually in very good preservation, though occasionally partly re- 
placed by a greenish mineral, doubtless some hydrous magnesian sili- 
cate, partly by a nearly colourless mineral (a hydrous potash mica ?). 
It is brown in colour, most markedly so when viewed normally to the 
basal pinacoid. Sections transverse to this plane are markedly 
dichroic, changing from an olive-brown to an almost colourless tint. 
These sections have a strong analyzing action. They have generally 
a border darker than the interior. In some specimens enclosures of 
other minerals are comparatively rare, in others rather abundant. 
These are grains and belonites of iron peroxide, and colourless 
needles (apatite ?), sometimes apparently smaller crystals of mica. 
Collins under a new name appears to be (as stated in the discussion after this 
paper by Mr. W. W. Smyth) only a variety of mica-trap. See also Mr. J. A. 
Phillips, Quart. Journ. Geol. Soc. vol. xxxi. p. 337. 
* There does not appear to be any essential difference between kersantite 
and kersanton. 
+ For list of localities where this rock is reported to occur, see Zirkel, 
‘Lehrbuch der Petrographie,’ vol. 1. p. 606. 
t For list of localities, see Zirkel, 2bid. vol. ii. p. 36, 
§ ‘Mikroskopische Physiographie,’ vol. ii. p. v. 
|| On the analogy of quartz-porphyry &c. one should say minette-porphyry ; 
but the term porphyry has been so vaguely applied that I venture to think 
it should be dropped altogether from scientific nomenclature, and quartz-felsite 
substituted for it.—T. G. B. 
