chap, xxxv MOVEMENTS OF MAGMA IN DYKE-FISSURES 15 1 
Jurassic series anywhere touched by the dyke is the Cornbrash. It is 
certain, therefore, that the igneous rock rises through all the subjacent mem- 
bers of the Jurassic series up to that horizon. There can be no doubt also 
that the Trias and Magnesian Limestone continue in their normal thickness 
underneath the Jurassic strata. To what extent the Coal-measures exist 
under Cleveland has not been ascertained ; possibly they have been entirely 
denuded from that area, as from the ground to the west. But the Millstone 
Grit and Carboniferous Limestone probably extend over the district in full 
development ; and below them there must lie a vast depth of Upper and 
Lower Silurian strata, probably also of still older Palaeozoic rocks and 
beneath all the thick Archaean platform. Tabulating these successive 
geological formations, and taking only tire ascertained thickness of each in the 
district, we find that they give the results shown in the subjoined table . 1 
STRATA CUT BY THE CLEVELAND DYKE 
Cornbrasli — 
Feet. 
Lower Oolite and Upper Lias, as proved by bore-hole on Gerrick Moor, . 950 
Middle and Lower Lias, ascertained from measurement of cliff-sections and 
from mining operations to be more than . . . .850 
Mew Bed Sandstone and Marl, found by boring close to the Tees to exceed 1,600 
Magnesian Limestone, at least . . . ■ • .500 
Coal-measures, possibly absent ...... 0 
Millstone Grit, not less than ...... 500 
Carboniferous Limestone series at least ..... 3,000 
Silurian rocks, probably not less than ..... 10,000 
17,400 
There is thus evidence that this dyke has risen through probably more 
than three miles of stratified rocks. How much deeper still lay the original 
reservoir of molten material that supplied the dyke, we have at present no 
means of computing. 
12. EVIDENCE AS TO MOVEMENT OF THE MOLTEN ROCK IN THE FISSURES 
It is usual to speak of the molten material of the dykes as having 
risen vertically within the fissures. And doubtless, on the whole, the 
expression is sufficiently accurate. In the case of such long dykes as those of 
Central Scotland and the North of England, where the petrograpliical 
character of the material remains so uniform throughout, it is obvious that 
the andesite or dolerite cannot have come from a mere single pipe like 
a volcanic orifice. Nor can we easily understand how it could have been 
supplied even from a series of such pipes. The general aspect and structure 
of the dykes suggest that the fissures were rent so profoundly in the crust 
of the earth as to reach down to a reservoir of molten rock which straight- 
way rose in them. The roof of such a reservoir, however, may have been 
irregular and uneven, so that a fissure need not have traversed it con- 
1 Drawn up for me by Mr. G. Barrow. 
