MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 653 
other hand use the term “ Bronzite.” There is therefore at present 
no general agreement on this point, and inasmuch as the test of 
pleochroism seems to show that the mineral of the Whin Sill is 
intermediate between enstatite and hypersthene, the term bronzite 
will probably not be considered inappropriate. 
Colourless monoclinic Pyroxene—In his paper on Swedish dia- 
bases Tornebohm* describes a colourless monoclinic mineral which 
he regards as a lime-magnesia bisilicate, and to which he gives the 
name Salit. It is distinguished from the prevailing pyroxene by the 
absence of colour, by the presence of more definite crystalline boun- 
daries, and by its easy alterability. In a few specimens of the 
Whin Sill I have observed a colourless monoclinic augite which may 
possibly be this mineral. It appears to be developed in the form 
of long slender prisms. Cross-sections show the forms o P, « P m, 
and oo P&, the pinacoids predominating just as in the exceptional 
variety of the common pyroxene above described. The individuals 
are, moreover, invariably twinned. I feel some doubt as to whether 
this is not, after all, a variety of the common pyroxene, which in the 
fresh condition and in a very thin section is frequently almost 
colourless. 
Hornblende.—This mineral is often present in small quantity. 
It never occurs as an independent mineral, but is always attached 
to the augite, from which it may be distinguished by its deep brown 
colour, its cleavages, and its pleochroism. The usual mode of oc- 
currence is in the form of small irregular patches on the margin of 
the augite. The boundary between the augite and the hornblende 
is abrupt but not sharp. The external boundaries of the hornblende 
are frequently formed of definite crystalline faces, and they project 
beyond the natural boundaries of the augite (see Pl. X XIX. fig. 3). 
The rélation therefore of the brown hornblende to the augite is not 
such as we should expect if the former mineral were produced from 
the latter by a process of paramorphic change. 
Mica.—A brown mica may occasionally be detected. It is by no 
means common, and when it does occur its characters are not such 
as to call for any special description. 
Quartz.—This substance occurs in the form of crystalline grains, 
and also in the micro-pegmatite. In the latter case it is of primary 
origin; in the former it may be, and probably is in many cases, of 
secondary origin. 
Apatite.—This occurs as dpi recognizable hexagonal prisms. 
Extremely long, thin, colourless needles are also abundant. These, 
I believe, are usually referred to apatite. 
Chlorite and other green Alteration-products.—In most sections. 
green fibrous minerals, resulting from the decomposition of the bisi- 
licates, may be observed, These have not been subjected to any 
exhaustive examination. 
Pyrite may be recognized both macroscopically and microscopi- 
cally. It is extremely irregular both in form and distribution. 
* Neues Jahrbuch, 1877, p 263. Many of the characters which Tornebohm 
gives as distinctive of the salit would apply to the bronzite, reas those 
depending on alteration. 
