Basalt. 
257 
in any way resembling the original rock. It must be 
always somewhat difficult to convince a beginner that 
this plastic clay is produced by the decomposition 
of basalt. 
But, a little patience will settle that point. As a 
matter of fact we can find basalt in every inter- 
mediate stage, from the fresh blue-black rock to the 
pasty clay referred to. We can go farther, and say 
that even if the clay had disappeared it might still be 
taken for granted that an Igneous rock once filled the 
fissures. This, we would conclude, from the fact that 
the basalt has left its mark, so to speak, on the sand- 
stones, by altering them to a harder rock. In many 
places around Sydney it is noted that the sand- 
stones are harder than usual, and show a rudely pris- 
matic structure. Stone of this class is much sought 
after for road-making. Whenever such a hardened 
sandstone is found it is promptly quarried, and, in the 
process of excavating, the cause of this hardening is 
usually discovered, in the shape of a dyke of Igneous 
rock whose contact altered and hardened the sand- 
stones. 
True, as already pointed out, this dyke is often 
altered almost beyond recognition ; the hard rock 
that once filled the fissures, in a liquid and molten 
condition, having now succumbed to the inexorable 
law of change. It will thus be seen why it is that 
baked and hardened sandstones are accepted as the 
work, direct or indirect, of intruded basalt. This is 
quite in keeping with all observed facts. It is also 
