562 
ELEMENTS OF GEOLOGY. 
first cut through by a greenstone dike, about 2^ feet wide; 
then the crack a, passed through all these rocks, and was 
filled up with quartz. The opposite walls of the vein are in 
some parts incrusted with transparent crystals of quartz, 
the middle of the vein being filled up with common opaque 
white quartz. 
We have seen that the volcanic formations have been called 
overlying, because they not only penetrate others but spread 
over them. M. Necker has proposed to call the granites the 
underlying igneous rocks, and the distinction here indicated 
is highly characteristic. It was, indeed, supposed by some of 
the earlier observers that the granite of Christiania, in Nor¬ 
way, was intercalated in mountain masses between the pri¬ 
mary or palaeozoic strata of that country, so as to overlie 
• fossiliferous shale and limestone. But although the granite 
sends veins into these fossiliferous rocks, and is decidedly 
posterior in origin, its actual superposition in mass has been 
disproved by Professor Keilhau, whose observations on this 
controverted point I had opportunities, in 1837, of verifying. 
There are, however, on a smaller scale, certain beds of euritic 
porphyry, some a few feet, others many yards in thickness, 
which pass into granite, and deserve, perhaps, to be classed 
as plutonic rather than trappean rocks, which may truly be 
described as in¬ 
terposed con¬ 
formably be¬ 
tween fossilifer¬ 
ous strata, as the 
porphyries (a, c. 
Fig. 616) which 
Fig. 616. 
Euritic porphyry alternating with primary fossiliferous divide the bitu- 
strata, near Christiania. . , , 
minous shales 
and argillaceous limestones, /*, f. But some of these same 
porphyries ^re partially unconformable, as 5, and may lead 
us to suspect that the others also, notwithstanding their 
appearance of interstratification, have been forcibly in¬ 
jected. Some of the porphyritic rocks above mentioned 
are highly quartzose, others very feldspathic. In propor¬ 
tion as the masses are more voluminous, they become more 
granitic in their texture, less conformable, and even begin 
to send forth veins into contiguous strata. In a word, we 
have here a beautiful illustration of the intermediate gra¬ 
dations between volcanic and plutonic rocks, not only in 
their mineralogical composition and structure, but also in 
their relations of position to associated formations. If the 
term “overlying” can in this instance be applied to a plu- 
