Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/222

 larger ones, very much compressed. These cells are lined with minute hexahedral prisms of quartz mixed with a greenish-yellow earthy matter, which is perhaps decomposed actynolite. This rock is penetrated by veins containing quartz, flesh-red jasper, and chalcedony, the latter of which fills all the cells of the adjacent rock.

An analogous variety is found on the top of the Wrekin, in which, however, the compression of the cells has proceeded so far as to bring their sides into actual contact, thus giving the rock a waved and striped appearance.


 * 2. Greenstone rocks.

These for the most part appear to lie under the claystone rocks. Their essential component ingredients are dull-green hornblende and greenish or reddish felspar. They all affect the magnetic needle, some of them in a very remarkable degree. They are more easy of decomposition than the felspar rocks just mentioned, and, in consequence, the respective place of each may be easily distinguished in the hills where they both occur, by the bare craggy surface of the one, and the smooth depressed verdant surface of the other.

When the component parts of the greenstone are distinct, and the felspar has its foliated crystalline structure, the only foreign ingredient which I have observed in the rock, is magnetic pyrites, in small grains. But where the hornblende and felspar are more intimately mixed, the rock usually becomes amygdaloidal, and contains globular concretions of felspar, quartz, calcareous spar, haematite, zeolite, and actynolite. Of these amygdaloids there is one of remarkable beauty, (described by Dr. Townson in the tract already referred to), and forming large masses on Caer Caradoc, but which has not yet found a place in the works of systematic mineralogists. It consists of a dull earthy basis, formed by an intimate mixture of dark bluish green hornblende, with flesh-red felspar, in closing globular concretions of