4 GEOLOGICAL SURVEY 
often of a different color from the rest of the mass, and hence the striking appearance of some varie- 
ties of this rock. 
Graphic granite is composed of a base of feldspar, traversed by variously shaped plates of quartz. 
When this rock is fractured in certain directions it presents the appearance of certain alphabetic 
characters. 
Albite granite. When felspar contains soda instead of potash it is called albite. When this 
mineral takes the place of common feldspar in granite, the rock is whiter but less durable than the 
common variety. 
Other minerals, such as hornblende, chlorite, tale, &c. often replace the ordinary constituents of 
granite. Where hornblende takes the place of the mica the rock is called Syenite, and when mica 
is also present with the hornblende it is called Syenitic granite. 
Protogine. This term is applied to those varieties of granite in which chlorite, tale, or steatite 
takes the place of mica. 
Granite is very widely distributed, being found in all parts of the known world. It rises in enor- 
mous masses, forming mountain peaks, and it is also found penetrating other rocks, in the form of 
veins. It every where underlies the older stratified rocks, and hence, in common with these, it has 
received the name Primary, on the supposition that it was the oldest or first formed rock. It is 
now, however, well understood to belong to various geological periods: for besides forming the great 
foundation of the earth, it is found piercing and superimposed upon strata of very modern date. 
This, however, has nothing to do with the origin of granite—it simply proves that the materials of 
which it is composed were in a fluid state; that in this form the mass was erupted or forced up- 
wards through the other rocks, even as late as the commencement of the 'Tertiary period, where it 
was consolidated, producing a granite in all respects as characteristic as that found beneath the old- 
est stratified rocks. Granitic veins are also seen to intersect granite. The apparent inconsistency 
of applying the term primitive or primary to a rock formed at so many and at such recent periods, 
induced Mr. Lyell to substitute the term Hypogene (formed beneath,) for that name. The term 
Primary, however, is so well understood, and in such common use, that but little advantage will 
arise from the change, particularly if we bear in mind the facts just stated. 
Basattic Rocks anp PoRPHYRIES. 
These rocks are far more limited in extent than the preceding ; instead of forming the base or 
frame-work of the earth’s crust, they occur rather in the form of dykes and veins, and appear to 
have been forced upwards through rents and fissures in the superincumbent rocks. They are, how- 
ever, widely distributed, and sometimes occupy considerable space. Basaltic rocks have been desig- 
nated by the names of whin, trap, greenstone, clinkstone, and amygdaloid. In general, they are 
more homogeneous in appearance, if we except the embedded minerals which they contain, and 
rarely present a crystaline structure like granite. Their color is a greyish black, approaching green. 
A remarkable difference between these rocks and granite consists in the greater abundance of lime, 
which is owing to the prevalence of hornblende in all the basaltic rocks. 
They occur in veins or dykes in tabular masses, as if spread out over the surface by overflowing. 
Basalt and trap often occur in beds composed of a series of vertical columns, exhibiting a struc- 
ture apparently the result of crystaline forces ; the sides presented by these columns vary in number 
from three to nine. This structure is not properly crystaline, but the effect of cooling after fusion, 
