SOME NORTH-OF-ENGLAND DYKES. 293 
stituents to assume a crystalline condition. Mr. Stock found (see 
his note, p. 225) that 35°57 per cent. of this rock remained insoluble 
after treatment with hydrochloric acid in a pressure-tube for one 
month, and that the insoluble residue yielded, on analysis, SiO, 
70°76 per cent.; Al,O, 10°93; FeO, 3°59; CaO 3:29; MgO 4:21; 
Alkalies 7-22. ‘This may be taken as representing approximately 
the composition of the base with its various devitrification-products. 
It is, as one would naturally expect, highly acid ; the silica here, as 
in other cases, having served as a sort of solvent medium out of which 
the other minerals have crystallized. 
So far I have been referring to the prevailing type of interstitial 
matter inthe Armathwaite slide. Here and there, however, in the 
same slide it puts on a very different aspect. It becomes deep 
brown in colour and is traversed by numerous ill-defined felspar 
microlites. The longulites and globulites are no longer to be recog- 
nized, and the whole patch gives a vague reaction on polarized 
light. This is the common type of devitrification, and may be 
well studied in sections taken from the rock which occurs at Bar- 
wick quarry, near Preston. Here the ill-defined felspar microlites 
occur in great abundance ; but as they have already been described 
it will be unnecessary to refer to them at greater length. In the 
thinnest possible sections the brown colour is seen to be due to a 
kind of indistinct granulation which has evidently been produced 
by the imperfect individualization of a brownish substance. ‘This 
substance appears as minute specks and ill-defined fibres. It is 
occasionally aggregated into irregular patches and narrow bands, 
which give the vivid polarization-tints characteristic of the small 
augite granules. Ihave not been able to recognize any clear trans- 
parent glass in the Preston slides; but some of the base containing 
indistinct fibres and granules appears isotropic, and may therefore 
be regarded as consisting of the micro-felsite of Rosenbusch. There 
is yet a third type of devitrification, also very common, which may 
be well studied in the specimens from Great Ayton. Indistinct 
granules, fibres, and ragged patches of a brownish substance which 
may be connected by intermediate forms with the augite granules, 
and which give the same reaction on polarized light, together with 
specks of opacite (? magnetite) lie intermixed with a substance which 
also reacts on polarized light. Thus, it splits up under crossed 
nicols into irregular patches of considerable size, which extinguish in 
definite positions, but whose boundaries are by no means well 
defined. Long, sharp, colourless microlites, and black hair-like 
bodies (trichites) may be observed in the base of the rock from 
certain localities. 
Structural Variations of the Rock. 
well-marked structural types can be recognized, one characteristic 
of the main mass, the other of the margins of the dyke. They differ 
from each other merely in the extent to which crystalline growth 
has been carried in the ground-mass of the rock; for the porphy- 
rite felspars are alike in both varieties. In the marginal part of 
the dyke the ground-mass is composed of minute felspar microlites 
