164 Proceedings of Royal Soeiety of Edinhurgli. [ feb . 20 , 
magma must liave been predisposed to consolidate as olivine-diabase 
with a great quantity of olivine. In the central portion of the 
large bodies, however, the igneous magma, as I explained above, 
acquired a greater acidity. Thus olivine could not separate, or the 
olivine crystals, already formed in the fluid magma at an earlier 
period, were corroded or wholly dissolved, so far at least as they 
were not preserved by a sudden consolidation of the whole mass. 
Such a sudden consolidation took place near the Salhands, and 
here the diabases are often exceedingly rich in well-defined oli- ! 
vine crystals. These suggestions may be further supported by 
the following details. The plagioclase-crystals in the heart of ' 
the more extensive diabase masses — as, for instance, from Auchen- , ; 
sterry, Hillwood Bath, Avoabridge near Linlithgow, St Margaret’s , ! 
Hope, Hound Point — show well-developed zonal structure, caused , 
by the acidity gradually increasing towards the centre of the ' “ 
crystal. Thus the extinction angles often differ in the different . j | 
zones of one crystal up to 40°. Although we must bear in mind , ; 
that the more basic minerals, as a rule, are the first to separate, 
and that the remaining magma thus gradually acquires a greater _ f 
acidity, I suppose this extreme variation in the acidity of the ; • 
zones of one felspar individual to signify that the acidity of the j . .. 
magma must have been absolutely increased by means of a real j 
addition of silica. In fact, I was able to prove by analysis that the ' 
diabase becomes more basic the nearer it approaches to the central j | 
portion. Further, I think it worth noticing that the immediate ' . i 
vicinity of the corroded olivine crystals scattered through the dia- ? ; 
base exhibits a specially fine-grained structure. This observation , f' j 
points, I think, to the conclusion that there is a chemical difference ^ ^ 
between the substance immediately surrounding the olivine and I 
that of the whole rock. i 
While the fresh diabases represent intrusive sheets invading the I 
Carboniferous sedimentary rocks of the basin of the Firth of Forth, \ 
there are also the so-called “white traps,” which, however, only i 
reach a thickness of 3 to 4 feet. At first sight, they seem to exhibit j' 
no similarity to diabase, t Colinswell, near Burntisland, however, f 
there occurs an intrusive sheet of 30 feet in thickness, which con- I' 
sists in its interior of normal diabase, while towards the outside the « | 
rock passes into “ white trap.” There appears to be evidence that 
