Transactions of the Royal Society of Edinburgh . s in thickness, they are almost invariably closer-grained. A cellularor amygdaloidal texture is hardly to be observed among them, and never wherethey are largely crystalline or granitoid. Differences of texture, however, mayoften be observed within short distances in the same mass, and likewise con-siderable varieties in colour and composition. As a rule, the most finelycrystalline portions are those along the junction with the stratified rocks, themost crystalline occurring in the central parts of the mass. A diminution inthe size of the crysta
Transactions of the Royal Society of Edinburgh . s in thickness, they are almost invariably closer-grained. A cellularor amygdaloidal texture is hardly to be observed among them, and never wherethey are largely crystalline or granitoid. Differences of texture, however, mayoften be observed within short distances in the same mass, and likewise con-siderable varieties in colour and composition. As a rule, the most finelycrystalline portions are those along the junction with the stratified rocks, themost crystalline occurring in the central parts of the mass. A diminution inthe size of the crystalline constituents may be traced not only at the base, butalso at the top of a sheet, or at any intermediate portion which has come incontact with a large mass of the surrounding rock. Salisbury Crags may be citedas a good example; another, and in some respects better, illustration is suppliedby the intrusive sheet at Hound Point (fig. 17), to the east of South Queensferry,where some layers of shale have been involved in the igneous rock, which. Fig. 17.—Intrusive sheets between shales and sandstones,Hound Point, Linlithgowshire. becomes remarkably close-grained along the junction.* This change in textureand absence of cellular structure form a well-marked distinction between thesesheets and those which have flowed out at the surface as true lava-streams. Another characteristic of the intrusive sheets is the alteration they produceamong the strata through which they have made their way, whether these lieabove or below them. The strata are sometimes crumpled up in such a way asto indicate considerable pressure. They are occasionally broken into fragments,though this may have been due rather to the effects of gaseous explosions thanto the actual protrusion of melted rock. But the most frequent changesuperinduced upon them is an induration which varies greatly in amount evenalong the edge of the same intrusive sheet. Sandstones are hardened intoquartz-rock, breaking with a smoot
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