166 Proceedings of Royal Society of Edinburgh, [fee. 20 ^ 
sition represents a slow gradation ; and I was thus able to show that 
network-skeletons always occur as the intermediate form between 
the irregular patches and the globulites. On the one hand, these 
skeletons increase regularly in quantity, while that of the ilmenite 
patches diminishes ; and, on the other hand, the former gradually 
diminish in favour of the globulitic grains. 
The apatite, which occurs in some specimens in large crystals 
visible to the naked eye, becomes a constituent of the ground-mass, 
where the rock assumes a microporphyritic structure. The olivine, 
however, continues to be separated out from the magma in large 
porphyritic crystals. According to these observations, we must 
conclude that the olivine belongs to an earlier period of consolida- 
tion than the apatite. 
In considering the modifications of the structure of the indivi- 
dual minerals, we have first to call attention to the plagioclase. 
In the interior of the larger diabase masses the plagioclase crystals 
seldom exhibit polysynthetic twinning; they are in most cases 
simple twins, or even totally untwinned crystals. In the same 
manner the lath-shaped crystals of the coarse-grained ophitic types 
are seldom composed of poly synthetic twin-lamellse. ITear the 
junction with the associated rocks, however, the great majority 
of the felspar crystals are finely twinned, often on two types. 
Besides the single twin-lamellse, which exhibit sharply-defined and 
entirely uncorroded forms, they sometimes appear to have been 
separated with intercalated interspaces between each other. The 
first impression given is, that the lamellae have yielded to a strain 
which operated on the crystal from without, and endeavoured to 
pull it asunder. Many lamellae really appear to have been drawn 
out of the crystal. 
As regards the augite, I have pointed out an analogous pheno- 
menon. Where the augites are sufficiently fresh to exhibit dis- 
tinct optical properties, they may all be recognised as untwinned 
crystals, if the specimen has been taken from the central part of 
a large massive bed. But the greater the proximity to the outside 
of the body the more does the quantity of untwinned augites 
diminish, while that of twinned individuals increases, till at last 
all the augites are twinned. I have expressed these data in the 
following table : — 
I 
s 
I 
i 
i 
