CHAP. XXXV 
ORIGIN AND HISTORY OF THE DYKES 
179 
Over the whole of the region traversed by the dykes, from the hills of 
Yorkshire and Lancashire to the remotest Hebrides, molten rock must have 
lam at a depth, which, in one case, we know to have exceeded three miles, 
and which was probably everywhere considerably greater than that limit. 
forced upwards, partly perhaps by pressure due to terrestrial contraction 
and partly by the enormous expansive force of the gases and vapours 
absorbed within it, the lava rose in thousands of fissures that had been 
opened for it in the solid overlying crust. That in most cases its ascent 
erminated short ot the surface of the ground may reasonably be inferred. 
t lc ' ast ’ we know, that many dykes do not reach the present surface, and 
that those which do have shared in the enormous denudation 0/ the 
surrounding country. That even in the same dyke the lava rose hundreds 
ot feet higher at one place than at another is abundantly proved. When, 
however, we consider the, vast number of dykes that now come to the light 
0 ay, and reflect that the visible portions of some of them differ more than 
f 000 feet froui each oth er in altitude, we can hardly escape the conviction 
that it would be incredible that nowhere should the lava have flowed out at 
the surface. Subsequent denudation has undoubtedly removed a great 
thickness of rock from what was the surface of the ground during older 
Tertiary time, and hundreds of dykes are now exposed that doubtless 
originally lay deeply buried beneath the overlying part of the earth’s crust 
through which they failed to rise. But some relics, at least, of the outflow 
°f lava might be expected to have survived. I believe that such relics 
remain to us in the great basalt-plateaux of Antrim and the Inner Hebrides. 
^ hese deep piles of almost horizontal sheets of basalt, emanating from 110 
great central volcanoes, but with evidence of many local vents, appear to me 
J lave proceeded in large measure from dykes which, communicating with 
10 surface of the ground, allowed the molten material to flow out in 
successive streams with occasional accompaniments of fragmentary ejections . 1 
, e tincture of the basalt-plateaux, and their mode of origin, will form the 
su !l ec t of the next division of this volume. 
We can hardly suppose that the lava flowed out only in the western 
egion oi the existing plateaux. Probably it was most frequently emitted 
ua accumulated to the greatest depth in that area. But over the centre of 
, dln j an( l of England there may well have been many places 
m< dm C jkeS aCtUally communica ted with the outer air, and allowed their 
o ° ea laatei 'ud to stream over the surrounding country, either from 
Aen fissures or from vents that rose along these. The disappearance of 
c | C 1 out fl°ws need cause no surprise, when we consider the extent of the 
.j/ 111 rttl °n which many dykes demonstrate. I have elsewhere shown that 
0ver Scotland there is abundant proof that hundreds and even thousands 
considfir!.V+] ter6Btln ? t0 "° te that in the great l’ a l ,er 0,1 P1, ysical Geology already cited, Hopkins 
fiuantitv f l e . f i u6stlon tlle outflow of lava from the fissures which he discussed. “If the 
"'ill of, 1 . , matter forced into these fissures,” he says, “be more than they can contain, it 
number of f 1Se ' ^ eje f e<1 ovor tlle aurfaoe 1 and if this ejection take place from a considerable 
the eiepT fri lhS " I ' es ’ and °7 er a tolerably even surface, it is easy to conceive the formation of a bed of 
matter of moderate and tolerably uniform thickness, and of any extent ” (op. tit. p. 71). 
