Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/186



The following simple minerals occur imbedded in the rocks of this series; they are enumerated in the order of their more frequent occurrence.

Granular olivine in disseminated grains seldom fails in the tabular basalt; and sometimes occurs also in greystone. It has a remarkable mode of decay, becoming tender, and iridescent.

Augite usually accompanies greenstone and sometimes greystone. Steatite is almost exclusively limited to the prismatic basalt; and it is likewise the only heterogeneous ingredient I have observed in it. It exists in small round specks, of a bottle-green colour so dark, that it is not easily discernible from the mass itself: it has a dull and waxy appearance; is soft, the streak greyish.

Glassy felspar characterizes the clinkstone porphyry. Calcareous spar, zeolite, iron pyrites, and chalcedony usually occur in association with the preceding substances.

Since the flœtz trap forms the superficial rock of the area we are now to describe, it may be proper to insert in this place a list of the most remarkable hills by which that surface is diversified, and some other remarks on its general forms. In the list of hills, the same order will be followed as before in tracing the chalk, beginning with the south-east angle and ending with the south-west. The thickness of the trap formation where it has been observed will be added.