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 be traced not only at the base, but also at the top of a sheet, or at any 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.[466] 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.
[466] See Hay Cunningham's "Essay," p. 66, and plate ix.; and Geol. Survey Memoir on "Geology of Edinburgh," p. 114.
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 occurrence of this structure in the coarser parts of the Whin Sill of the north of England.[467] It occurs also in a marked degree in the Ratho sill and in some portions of the great doleritic sill of which the crags of Stirling form a part.[468]
[467] British Petrography, p. 208.
[468] Mr. H. W. Monckton. Quart. Journal Geol. Soc. vol. li. (1895), p. 482.
But beside the differences in texture, mainly due to varying rates of cooling, the sills sometimes exhibit striking varieties of composition in the same mass of rock. These variations are more especially noticeable among the larger sills, and particularly where the material is most markedly basic. The special type of differentiation, so noticeable in the Bathgate diabase and picrite mass already alluded to, is likewise well exhibited in an intrusive sheet or group of sheets, recently exposed at Barnton, in the cutting of a railway from Edinburgh to Cramond[469] ([Fig. 161]). The intrusive nature of the several bands of igneous rock which occur here is made quite evident by the alteration they have produced upon the shales with which they have come in contact. It is the uppermost and most extensive of these sills which specially deserves notice, for the differentiation of its constituents. It stretches along the cutting for several hundred yards at an angle of dip of about 15°. At the western or upper part of the mass its actual contact with the superincumbent sedimentary strata is not visible, but as the igneous rock is there a good deal finer in grain than elsewhere, its upper surface cannot be many feet distant. The upper visible portion is a light well-crystallized dolerite with a rudely bedded structure, the planes dipping westwards at 15°. About 20 or 30 feet below the upper visible termination of the mass, the dark ferro-magnesian minerals begin rapidly to increase in relative proportion to the pale felspar, and the rock consequently becomes dark-greenish brown. The change is particularly noticeable in certain bands which run parallel with the general dip. There is no definite line between the pale and dark body of the rock, the two graduating into each other and the darker part becoming deeper in colour, heavier and more decomposing, until it becomes a true typical picrite. Even in this ultra-basic portion the same rude bedding or banding may be observed.
[469] This rock has been described by Mr. J. Henderson and Mr. Goodchild, Trans. Geol. Soc. Edin. vi. (1893) pp. 297, 301, and by Mr. H. W. Monckton, Quart. Journ. Geol. Soc. l. (1894) p. 39. Mr. Goodchild recognized the occurrence of picrite, and Mr. Monckton has described the succession of rocks, and given a diagram of them.
Fig. 161.—Section of Sill, Cramomd Railway, Barnton, near Edinburgh.
1. Baked shale; 2. Sill of very felspathic dolerite about, nine feet thick; 3. Baked shale, eight inches; 4. Dolerite showing chilled fine-grained edge and adhering firmly to the shale below; it rapidly passes up into (5) Picrite with white felspathic veins (6); 7. Junction of picrite and dolerite with a similar vein along the line of contact; 8. Large globular body of dolerite enclosing a mass of picrite.
Veins in which felspar predominates over the darker minerals traverse the rock, sometimes parallel with the bedding, sometimes across it. They vary from less than an inch to a foot in width, sometimes dividing and enclosing parts of the surrounding mass. But that they are on the whole contemporaneous with the sill itself, and not long subsequent injections, is shown by the way in which the dark ferro-magnesian minerals project from the picrite into the veins and lock the two together.