The third coarse variety, or dolerite, is perhaps the most abundant. It is the rock which yields the boulders by exfoliation, and seems to harden as it approaches the kernel. It occurs often at the foot of the hills, and often towards the summit, to which by its decomposition it gives a conical appearance. Such hills are difficult of ascent, as they are steep and yielding to the tread; and similar high inclinations, due to the more rapid decomposition of the same rock, may be generally seen below the harder scarps of the flat-topped hills. The unaltered spheroidal cores are sometimes seen at the foot of the hills, or scattered over the surface of the bed from which they have weathered out. Porphyritic basalt is not much represented; and columnar structure is very seldom seen. The basalt in dikes is generally disposed in vertical segments of small dimensions.