Page:A Treatise on Geology, volume 1.djvu/164

 148 distribution of organic remains in it is in many respects very similar (Hall, Geol. of New York).

Physical Geography.—Lying on the sloping sides of the slate systems of Wales and Cumberland, the principal masses of the Silurian rocks show but little boldness of feature, compared to these older rocks: frequently, as in the beautiful neighbourhood of Ludlow and Aymestry, and in the Vale of the Towy, they are richly covered with woods, as ancient, perhaps, as Caractacus. The limestone country about Dudley is pleasingly varied.

Igneous Rocks.—These are abundantly exhibited in irruptive axes and points throughout the Silurian formations. Lilleshall Hill, the Wrekin, and other points about it, consist of compact felspar, ranging N.E. and S.W., and they convert sandstones into quartz rock. Caer Caradoc, the Lickey, Helmeath, &c. constitute a similar and parallel group of hills, in which greenstone, actynolitic trap, &c. occur: similar changes happen to the sandstones which touch the trap: the argillaceous rocks are indurated and much altered. The Breiddin group of hills consists of porphyries, compact felspar, greenstone, &c.; and near these the strata of the Silurian system are indurated and fissured. The same range (N.E. to S.W.) is noticed in the trap rocks near Old Radnor, Builth, and Baxter's Bank, near Llandrindod. Hypersthene abounds in traps near Old Radnor, and great changes happen to the Ludlow and Wenlock rocks near them: limestone becomes crystallised; shale is indurated; anthracite, copper ore, iron pyrites, and bad serpentine, are generated at the contact. The large trap district of Llandegley, Llandrindod, and Builth, presents a variety of such phenomena, and the mineral springs of Builth, Llandrindod, &c., are supposed to be residual effects of the same igneous agency. In Brecknockshire and Caermarthenshire, similar phenomena are repeated around several erupted masses of trap.

The Malvern hills consist of Silurian rocks resting on a granito-syenitic base, which has been uplifted with