560 
ELEMENTS OF GEOLOGY. 
a dike, such as intersect the tuffs and 
lavas of Vesuvius and Etna. Dikes 
of granite may be seen, among other 
places, on the southern flank of Mount 
Battock, one of the Grampians, the 
opposite walls sometimes preserving 
an exact parallelism for a considera¬ 
ble distance. As a general rule, 
however, granite veins in all quarters 
of the globe are more sinuous in their 
course than those of trap. They pre¬ 
sent similar shapes at the most north¬ 
ern point of Scotland, and the south¬ 
ernmost extremity of Africa, as the 
annexed drawings will show. 
It is not uncommon for one set of 
granite veins to intersect another; 
and sometimes there are three sets, as in the environs of Hei¬ 
delberg, where the granite 
on the banks of the river 
NTecker is seen to consist of 
three varieties, differing in 
color, grain, and various pe¬ 
culiarities of mineral com¬ 
position. One of these, 
which is evidently the sec¬ 
ond in age, is seen to cut 
through an older granite; 
and another, still newer, 
traverses both the second 
and the first. In Shetland 
there are two kinds of gran¬ 
ite. One of them, composed of hornblende, mica, feldspar, 
and quartz, is of a dark color, and is seen underlying gneiss. 
The other is a red granite, which penetrates the dark variety 
everywhere in veins.J 
Fig. 614 is a sketch of a group of granite veins in Corn¬ 
wall, given by Messrs. Von Oeynhausen and Von Dechen.§ 
The main body of the granite here is of a porphyritic appear¬ 
ance, with large crystals of feldspar; but in the veins it is fine¬ 
grained, and without these large crystals. The general height 
of the veins is from 16 to 20 feet, but some are much higher. 
* Captain B. Hall, Trans. Eoy. Soc. Edinburgh, vol. vii. 
t Western Islands, pi. 31. 
i MacCulloch, Syst. of Geol., vol. ii., p. 58. 
§ Phil. Mag. and Annals, No. 27, New Series, March, 1829. 
Fig. 613. 
Fig. 612. 
