CHAP, xxvn 
SILLS, BOSSES AND DYKES OF THE BUYS 
451 
change as occurs at the t(jp of the mass. There the felspar rapidly begins 
to predominate over the darker minerals, and the dolerite into which the 
rock passes shows a fine-grained margin adhering firmly to the shales on 
which it rests. This lower doleritic band, showing as it does the effect of 
chilling upon its under surface, may he duo to more rapid cooling and 
crystallization, while in the overlying parts the mass remained sufficiently 
mobile to allow of a separation of the heavier minerals from the felspars, 
which appear in predominant quantity towards the top. It must he frankly 
admitted, however, that we are still very ignorant of the causes which 
led to this separation of ingredients in a few sills, while they were entirely 
absent or non-efficient in most of them. 
The intrusive character of the Carhoniferous sills of Central Scotland 
and their contact-metamorphism have been fully described, and some of 
them have become, as it were, “ household words ” in geology.^ Exposed in 
so many fine natural sections in the vicinity of Edinburgh, they early 
attracted the notice of geologists, and furnished a battle-ground on which 
many a conflict took place between the Phitonist and Xeptunist champions 
at the heginniug of the present century. 
As the sills frequently lie in even sheets perfectly parallel with the 
bedding of the strata between which they have been injected, care is required 
ill some cases to establish that they are of intrusive origin. One of the 
most obvious tests for this jiurpose is fumished l>y the alteration they 
produce among the strata through which they have made their way, 
whether these lie above or below theni. The strata are sometimes crumpled 
up in such a manner as to indicate considerable pressure. They are occa- 
sionally 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 quartzite, breaking with a 
smooth clear glistening fractui'e. Coals are converted into a soft sooty 
substance, sometimes into anthracite. Limestones acquire a crystalline 
saccharoid structure. Shales pass generally into a kind of porcellanite, but 
occasionally exhibit other types of contact-nietaniorphism. Thus below the 
thick pici’ite sill at Barnton, near Edinburgh, the shales have assumed a 
finely concretionary structure by the appearance in them of spherical pea-like 
aggregates. 
Another proof of intrusion is to be found in the manner in which sills 
catch up and completely enclose portions of the overlying strata. The 
well-known examples on Salisbury Crags (Eig. 162) are paralleled by scores 
of other instances in different parts of the same region. 
^Moreover, sills do not always remain on the same horizon ; that is, 
1 See, for iustaiice, llaolaveii’s Geoloc/y of Fife and ilie. Lolhians, 1839 ; Hay Cunningham’s 
Essay, previously cited ; Oe.oloyiml Survey Memoir on. the Geology of Edmbnryh (Slieet 32), 1861 ; 
Mr. Allport, Quart. Joiirn. C'col. Soc. vol. xxx. (1874) p. 553 ; Teall, BriUsh Fetrography, p. 1S7 ; E. 
Steelier, Cmitartorscheimingcn an schoitischen Olitindiahusen, Tschennak’s Miiieralog. Miitheil. vol. 
ix. (1887) p. 145 ; Froc. Boy. Soc. Edin. vol. xv. (1888) p. 160. 
