
Parr VII. Snot. i.§ 2.) ERUPTIVE SHEETS. 549 
- dolerite have been much hardened, the shales being converted into a kind 
of porcellanite, and the sandstones into quartzite. The dolerite in the 
centre of the bed is a coarse-grained rock, in which the component 
minerals can readily be detected with a lens, or even with the 
_ unassisted eye. But as it approaches the sedimentary beds, above and 
below, it becomes finely crystalline. Ihave had sections cut for the micro- 
scope, showing the actual junction of the two rocks. (See Fig. 25, p. 148.) 
In these it is interesting to observe that the dolerite, for about the 
eighth of an inch inwards from its edge, consists mainly of an altered 
glass in which lie well-formed crystals of triclinic felspar and numerous 
opaque tufted microliths, which may be of augite. An inch back from 
the edge the glass and the microliths have alike disappeared, and the 
rock is merely a crystalline dolerite, though finer in grain than in the 
central portions of the bed. Numerous steam or gas vesicles occur in 
the vitreous part, some of them empty, but mostly filled with calcite or a 

0) “in 
Fic. 276.—Mass oF SANDSTONE AND SHALE (a) IMBEDDED IN THE DOLERITE (6) oF 
SALIsBuRY ORAGS, AND INJECTED WITH VEINS AND THREADS OF IT. 
brown ferruginous earth. ‘There can be little doubt that the vitreous 
structure of this marginal film was originally that of the whole rock. 
The thinness of the glassy crust is in harmony with all that is known 
as to the feeble thermal conductivity of lava. When the dolerite was 
- intruded it was no doubt a molten glass containing much absorbed 
vapour, the escape of which at its high temperature was probably the 
main agent in indurating the adjacent strata. In a number of slices cut 
_ from different parts of the central portion of the dolerite, I have failed 
to detect any of the steam-holes so marked in the outer vitreous edge. 
The retention of this absorbed vapour in the general mass of the molten 
rock doubtless facilitated the process of crystallization from the original 
glassy condition. 
This greater closeness of texture at the surfaces of contact forms 
one of the distinguishing marks of an intrusive as contrasted with a 
contemporaneous sheet (p. 563). Microscopic examination of these 
1 Mr. Sorby has observed in specimens from this locality sliced by him for micro- 
scopic examination that the fluid cavities in the quartz grains have been emptied.— 
“ Address,’ Q. J. Geol. Soc, xxx. 
