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MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 645 
augite grains one now recognizes collections of such grains, and also 
definite crystalline plates into which the lath-shaped felspars fre- 
quently penetrate. In short the rock possesses the “ ophitic structure” 
of Messrs. Lévy and Fouqué. The interstitial matter becomes more 
pronounced and takes on here and there the micropegmatitic struc- 
ture. In the coarser-grained varieties both the ophitic and micro- 
pegmatitic structures become more and more developed, and the 
dominant pyroxene shows a péculiar striated appearance, which 
will be described more fully in another part of this paper. In the 
coarsest variety of rock the pyroxene is developed in the form of long 
and somewhat flattened prisms and the micropegmatitic structure 
is most strikingly shown. Having described the general types of 
micro- and macro-structure, it will now be advisable to consider the 
individual minerals. 
Felspar.—This is probably, in all cases, the most abundant mine- 
ral. In the vast majority of cases only one generation can be 
detected. In three specimens, however, taken from Little Mill near 
Alnwick, Middleton in Teesdale, and Barrasford on the North Tyne, in 
addition to the prevailing felspars which give lath-shaped sections, a 
larger felspar, which is more evenly developed in the different direc- 
tions, and which contains a nucleus with irregular ramifying inclu- 
sions, may be recognized. ‘This evidently belongs to an earlier 
period of consolidation, and the rock is therefore not strictly granular 
in the sense in which that term is used hy Rosenbusch *. The pre- 
vailing felspar, as already indicated, gives lath-shaped sections. When 
fresh it shows the fine striation of plagioclase. Whether it belongs 
to one or more species I am not able to determine with any degree of 
confidence. ‘The extinction-angles referred to the twinning-plane 
are not large. In the majority of cases the felspar is more or less 
altered ; and this alteration appears to affect all the individuals alike, 
a fact which tells in favour of the view that we are here dealing 
with a single species. For the purpose of isolating the different 
constituents a specimen of a moderately coarse-grained variety of 
the normal whin was broken up in an iron mortar, and the frag- 
ments passed through a series of sieves. The grains selected for 
further treatment were those which passed through a sieve having 
50 meshes to the inch and were stopped by one having 100 meshes. 
These grains were so small as to consist, in a large measure, of the 
individual constituents of the rock. They were washed to remove 
the fine powder, dried and then placed in the Sonstadt solution 
having a specific gravity of about 3. The grains of pyroxene and 
titaniferous magnetic iron sank to the bottom, whereas the grains of 
felspar substance and many others of a composite character remained 
suspended. The grains of pyroxene and magnetic iron-ore were 
separated for further treatment, and then the solution was diluted 
down gradually. It was found that the felspar substance remained 
suspended in a solution of 2°70, and sank in one of 2°67. The 
grains which fell as the solution was diluted from 2°70 to 2-67 
* ‘Ueber das Wesen der kornigen und porphyrischen Structur bei Massen- 
gesteinen,’ Neues Jahrbuch, Band ii. 1882. 
Oe Ga. No. Le: : 2x 
