48 
VOLCANIC EOCKS, 
existing volcanic v.ent, as to render it probable 
that the latter also have been poured forth from 
the interior of the earth. We further find the 
rocks adjacent to volcanic craters, intersected by 
rents and fissures, which have been filled with 
injections of more recent lava, forming trans- 
verse walls or dykes. Similar dykes occur not 
only in districts occupied by basalt and trap 
rocks, at a distance from the site of any modern 
volcanic activity ; but also in strata of every 
formation, from the most ancient primary, to 
the most recent tertiary (see Plate 1. section 
f 1 — f 8. h 1 — h 2. i 1 — i 5) : and as the mineral 
characters of these dykes present insensible gra- 
dations, from a state of compact lava, tlirough 
infinite varieties of greenstone, serpentine, and 
porphyry to granite, we refer them all to a 
common igneous origin. 
The sources from which the matter of these 
ejected rocks ascends are deeply seated be- 
neath the granite ; but it is not yet decided 
whether the immediate cause of an eruption 
be the access of water to local accumulations 
of the metalloid bases of the earths and alkalies ; 
or whether lava be derived directly from that 
general mass of incandescent elements, which 
may probably exist at a depth of about one hun- 
dred miles beneath the surface of our planet.* 
* See chapter on the internal temperature of the earth. 
