CHAP. XXVII SILLS, BOSSES AND DYKES OF THE BUYS 
they are largely crystalline. This texture is most often to be found in thin 
sills which have been injected among carbonaceous shales or coals, These 
intrusive sheets are generally finely cellular, and more or less decayed 
(“ white trap ”). 
Differences of texture may often be observed within short distances in 
the same sill, and likewise considerable varieties in colour and composition. 
The most finely crystalline portions are, as usual, those along the junction 
with the stratified rocks, the most crystalline occurring in the central parts 
of the mass. A diminution in the size of the crystalline constituents may 
he traced not only at the base, but also at the top of a sheet, or at any 
Fig. 160.— Sills between shnles luul sauilstoiie.s, Homnl Point, Linlithgowshire. 
intermediate portion which has come in contact with a large mass of the 
surrounding rock., A good illustration is supplied by the intrusive sheet at 
Hound Point (Fig. 160), to the east of South Queensferry, where some 
layers of shale have been involved in the igneous rock, which becomes 
remarkably close-grained along the junction.^ This change in texture and 
absence of cellular structure form a well-marked distinction between these 
sheets and those which have flowed out at the surface as true lava-streams. 
Some of the larger doleritic sills display a somewhat coarsely crystalline 
texture in their central portions, and occasionally present a notable 
micropegmatitic aggregate, which plays the part of interstitial substance 
enclosing the other minerals. Mr. Teall has referred to the frequent occur- 
rence of this structure in the coarser parts of the Whin Sill of the north of 
England.^ It occurs also in a marked degree in the Eatho sill and in some 
portions of the great doleritic sill of which the crags of Stirling form a part.® 
But beside the differences in texture, mainly due to varyimi rates of 
cooling, the sills sometimes exhibit striking varieties of composition in the 
same mass of rock. These variations are more especially noticeable amono- 
the larger sills, and particularly where the material is most markedly basia 
The special type of differentiation, so noticeable in the Bathgate diabase and 
picrite mass already alluded to, is likewise well exhibited in an intrusive 
slieet or group of sheets, recently exposed at Baruton, in the cuttiim of a 
railway from Edinburgh to Cramond ^ (Fig. 1 61). The intrusive nature'of the 
■ See Hay Cunningham's “ Ess.ay,” p. 66, and plate ix. ; and Geol. Survey Meimir on “ Geology 
of Edinburgh,” p. 114. ^ British Petrography, p. 208. “ 
® Sir. H. AV. Honokton. Quart. Journal Geol. Soc. vol. li. (1895), p. 482. 
■* This rock lias been described by Mr. J. Henderson and Mr. Goodehild, Trans. Geol Soc 
Edin. vi. (1893) pp. 297, 301, and by Mr. H. W. Monekton, Quart. Journ. Geol Soc 1 (1894) 
VOL. 1 . 2 G 
