

194 STRUCTURAL AND FIELD GEOLOGY 
The strata in contact with a sill never fail to afford 
evidence of having been subjected to the action of heat. 
Both overlying and underlying strata are invariably affected, 
the alteration at the point of contact being often excessive. 
But the alteration never extends so far from the eruptive 
rock as in the case of granitic intrusions. Some account of 
these and other changes produced by sills will be considered 
in the sequel. 
Evidence is not wanting to show that sills have now and again melted 
up and absorbed some of the rocks with which they have come in 
contact. In the Scottish coal-fields, for example, they have not infre- 
quently eaten up thick seams of coal and black shale which they have 
followed as lines of least resistance. In cases of this kind the basalt- 
rock is usually much altered, becoming bleached white or yellow, and 
assuming a dull, clay-like aspect (“white trap”). Limestones are occa- 
sionally demolished in the same way, and their place taken by sills. 
But to what extent other kinds of rock may have been absorbed is quite 
uncertain. The rock of a thick sill not infrequently varies in petro- 
graphical character, being in some places less basic than the normal. 
But while such variations may be the result of absorption of extraneous 
materials, they seem just as likely to be due to magmatic differentiation, 
the more basic areas having separated out during the earlier stages of 
cooling. It may be mentioned, however, that not infrequently the intrusion 
of a sheet into a series of strata lying between two seams of coal or 
ironstone has not apparently increased the distance between those seams, 
as it might have been expected to do. In the neighbourhood of 
Dalmellington, Ayrshire, for example, thick sheets of basalt have been 
here and there intruded amongst a series of sandstones and shales 
which come between two conspicuous seams—a coal and a blackband 
ironstone—the distance from the one seam to the other being quite 
well known. In some places only one sheet is present; in other parts 
of the same neighbourhood there are two, while at intermediate points 
the pits may encounter none at all. Yet, in sinking shafts, the miners 
always reach the seam (ironstone) they are in search of at the estimated 
depth below the coal, no matter whether sills are present or not. In 
short, the distance between the two given horizons is neither increased 
nor diminished by the presence or absence of the intrusive sheets. It 
seems difficult to account for such phenomena (and many similar instances 
occur), except on the supposition that molten rock has the power of 
absorbing rock-material, and that, as Mr Clough has suggested, there 
may have been a general circulation in the mass which reduced all parts 
of the mixture to a uniform composition. But much petrographical and 
chemical research must be done before a question of this kind can be 
settled satisfactorily. 
Sills often appear in large numbers in regions of former volcanic 
activity. Those associated with the Carboniferous strata of Scotland are 


