STRUCTURE OF THE TITTERSTONE CLEE HILL. 
221 
deposited under water—these lie terms Volcanic ; and 2ndly, 
those which have been formed at immense depths below the 
surface, which he names Platonic. 
We have before us an example of the first. Of the 
volcanic class of rocks there are again two main divisions, 
the basaltic and the trachytic ; the first of these are dis¬ 
tinguished by the comparative smoothness of their texture 
and their breaking with a conclioidal fracture, while the 
trachytic are rough to the touch owing to the quantity of 
granular felspar which usually enters into their composition. 
There is, besides, an important difference in the constituents 
of them, since, while basalt contains silica or quartz in but 
small quantity, i.e., something under 50 per cent., trachyte is 
much more rich in that mineral ranging up to 60 and 70 per 
cent. And here I may observe that a corresponding difference 
exists in the two classes of plutonic rocks which correspond 
to those of the volcanic. Syenite bears much the same 
relation to basalt which granite does to trachyte. The 
former is often entirely deficient in silica, whereas in 
granite its presence is conspicuous. While, however, these 
distinctions hold as a general rule, a transition from one 
class to another may often be observed. 
The term trap is derived from a Swedish word, trappa, 
which means a flight of steps ; and has been applied to this 
form of igneous rock from the fact that it frequently occurs 
in successive terraces, rising above each other in the form of 
steps. Some seven minerals enter into its composition, viz., 
silica, alumina, iron, magnesia, lime, potash, and soda. The 
proportion of these vary considerably, as they do in modern 
lavas, even in those which have been ejected from the same 
crater. To account for this fact it has been suggested that 
when a volcano is in action the lighter portions of the molten 
rocks, the felspatliic trachytes, are first ejected; while the 
tcasor, or the basalt, is the last to make its appearance, 
issuing in streams pushed through the superincumbent strata, 
as here. 
In many places basalt is seen to assume a distinctly 
columnar form. This may be observed in those parts of the 
Clee Hills which are quarried, as well as in the shapes of the 
numerous blocks which lie around. The number of angles of 
these prisms varies from three to twelve, but is commonly 
from five to seven. The dimensions of these columns are 
very variable. Sometimes they attain a length of 400 feet, 
sometimes of only an inch or two ; their diameter also vary¬ 
ing from 9 feet to an inch. It is a remarkable fact that their 
axis is always perpendicular to the surface on which the lava 
