CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 475 
their mass, so far at least as that where they occur in beds only two or three 
feet or yards in thickness, they are almost invariably closer-grained. A cellular 
or amygdaloidal texture is hardly to be observed among them, and never where 
they are largely crystalline or granitoid. Differences of texture, however, may 
often be observed within short distances in the same mass, and likewise con- 
siderable varieties in colour and composition. As a rule, the most finely 
crystalline portions are 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. Salisbury Crags may be cited 
as a good example; another, and in some respects better, illustration is supplied 
by 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. 
Hound Point, Linlithgowshire. 
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. 
Another characteristic of the intrusive sheets is the alteration they produce 
among the strata through which they have made their way, whether these lie 
above or below them. The strata are sometimes crumpled up in such a way as 
to indicate considerable pressure. They are occasionally broken into fragments, 
though this may have been due rather to the effects of gaseous explosions than 
to the actual protrusion of melted rock. But the most frequent change 
superinduced upon them is an induration which varies greatly in amount even 
along the edge of the same intrusive sheet. Sandstones are hardened into 
quartz-rock, breaking with a smooth clear glistening fracture. Shales pass into 
,,.a kind of porcellante, jasper or Lydian-stone. Coals are converted into a soft 
sooty substance, sometimes into anthracite. These alterations, and the 
remarkable changes of texture experienced by the invading dolerites, will 
be again referred to in Part IT. 
Further evidence of the truly intrusive nature of these sheets is to be 
* See Hay Cunnineuam’s “ Essay,” p. 66, and plate ix., and “Geol. Survey Memoir on Geology 
Edinburgh,” p. 114. 
