STRUCTURE OF ERUPTIVE ROCKS 193 
fragments torn from the contiguous rocks; occasionally, 
indeed, large slabs or sheets of the invaded strata have been 
caught up and enclosed in the eruptive rock, and such 
fragments are invariably much baked and altered. Many 
thick sills divide into two or several subordinate sheets, 
each more or less closely following a plane of bedding. Often, 
also, dykes and veins proceed from sills into the adjacent 
rocks. This is frequently the case when a thick sill divides, 
the separate sheets being often connected by one or more 
dykes passing across the intervening strata. But the whole 
complex of sheets and dykes has obviously been intruded 
at one and the same time. Each independent sill or group 

Fic. 67.—SILL OR INTRUSIVE SHEET. 
b, dolerite ; s, sandstones and shales. 
of subordinate and associated sheets is doubtless connected 
with one or more vertical pipes or feeders, although these 
have not often been seen in section. 
The sills of our Carboniferous areas consist principally 
of basic rocks, mainly dolerites and basalts. Some of these 
are not more than a few feet or yards in thickness; others 
may reach and even exceed 150 feet. They are all lenticular 
in shape, some dying out more rapidly than others. At and 
near its junction with the overlying and underlying strata, 
a sill is almost invariably finer grained than towards the 
centre of the mass. Along the actual line of contact it is 
frequently compact and even markedly vitreous. In the 
case of thin sheets the texture is usually finer grained, and 
the rock may contain much glassy base throughout. The 
thicker sills, on the other hand, tend to be coarser grained 
and holocrystalline. Vapour cells are usually absent, although 
now and again sporadic areas of vesicles appear; but these 
are never so plentiful as to impart a scoriaceous aspect to 
the rock. 
N 

