£92 BED CONTAINING FOSSIL SHELLS 
of its planes, the composition being identical, and, without 
doubt, contemporaneous. These changes and passages of 
one rock into the otlier, are so frequent and various, as to 
render it difficult to refer the most of them to either of the 
rocks I have above mentioned as types. I shall therefore 
proceed to describe those which are distinctly marked, and 
their accompanying minerals. In external appearance, the 
columnar and semi-columnar basalt closely resembles that 
of the Giant's Causevvay, possessing the same fracture, in- 
ternal dark colour, and external brown crust. It is equally 
compact and sonorous. It, however, contains more fre- 
quently crystals of olivine, of basaltic hornblende, and of 
carbonate of lime. The fusibility of each is the same. Per- 
haps the basalt of the Gawilghur Range more nearly re- 
sembles in everyrespeet that of the Pouce Mountain in the 
Mauritius. This is, however, of very little importance, 
since every body who has travelled much in trap countries, 
knows well what great changes in composition and structure 
occur even in continuous masses. Among the minerals, 
calcedony and the different species of zeolite, are rarely- 
found in the columnar basalt, but they are of frequent oc- 
currence in that which is semicolumnar. 
The wacke or indurated clay is as various in character and 
composition as the basalt, and, unfortunately, I have no type 
with which to compare it, as in the case of the basalt of the 
Giant'*s Causeway. Its colour varies with its constituents, 
but is most usually grey. It is easily frangible, very fre- 
quently friable, and is almost always porous and amygda- 
loidaL It appears to be composed of earthy felspar and 
hornblende, with a considerable proportion of oxide of iron. 
It is always easily fusible into a black scoria or glass, ac- 
cording with the quantity of zeolite which it contains. Of 
dll the trap rocks it abounds the most in simple minerals* 
