644 J. J. H, TEALL ON THE CHEMICAL AND 
less altered, a monoclinic pyroxene haying certain special characters 
hereafter to be described, and titaniferous magnetic iron-ore. In the 
coarser-grained varieties quartz may always be detected as an 
original, and probably also as a secondary, mineral. The original 
quartz is intergrown with felspar so as to produce the micro-peg- 
matitic structure of Lévy and Fouqué, or the granophyric structure 
of Rosenbusch. ‘This micro-pegmatite plays the role of ground-mass, 
that is, it occurs between the interstices of the lath-shaped felspars. 
If a series of sections cut from rocks which vary in the size of the 
constituent minerals be examined, it will be seen that the micro- 
pegmatitic structure in the interstitial matter becomes less and less 
pronounced as the finer-grained varieties are observed, until at last 
it is incapable of definite recognition; all that one then sees with 
the highest powers is an indistinct parallel fibrous structure. It is 
extremely interesting to trace this micro-pegmatite into a substance 
in which the minerals quartz and felspar are incapable of recog- 
nition by the aid of the microscope, but in which there is reason to 
believe that they exist. 
Apatite can always be detected, though only in very small quan- 
tity. 
The accessory minerals, or those which do not always occur in the 
thin sections, include a rhombic pyroxene, a colourless monoclinic 
pyroxene—the so-called salit of many authors—brown hornblende, 
mica, pyrite, calcite, and various green decomposition-products. 
Before proceeding to describe the characters of the individual con- 
stituents it will be well to say a few words about the general 
microscopic structure of the different varieties. 
The compact rock which forms, in many cases, the actual junction, 
and which may be well seen at both the upper and under surfaces 
of the Whin Sill in the neighbourhood of Cauldron Snout, shows a 
micro-porphyritic structure. Small, more or less lath-shaped sec- 
tions of felspar lie imbedded in a ground-mass, which is a veritable 
“ Mikrolithenfilz,” composed of minute and evenly distributed specks 
of opaque iron-oxide and somewhat ill-defined and extremely small 
colourless micrclites. In the thinnest preparations these microlites 
are seen superposed on each other, so-that it becomes impossible to 
speak with certainty as to the presence or absence of any isotropic 
glass. Itisimpossible to look at this ground-mass and not be struck 
with its resemblance to the ground-mass of many andesites and por- 
phyrites. 
The finely crystalline rock which occurs a foot or two from the 
junction presents a very different type of micro-structure. It appears 
at the first glance to be holocrystalline, and to consist of crippled 
augite granules, lath-shaped felspars, and grains of opaque iron-ore. 
On closer examination an extremely minute quantity of interstitial 
matter may generally be detected between some of the lath-shaped 
felspars. 
The medium-grained rock which makes up the main mass of the 
Whin Sill differs from the above merely in the extent to which the 
individual constituents have developed. Instead of the crippled 
