From the Upper Ludlow rocks, if they exist above these, no organic remains have yet been collected.
The chief peculiarity in the Silurian strata in this district is the separation of the Wenlock limestone into two bands. The same fossils occur in both bands and in the intermediate thick band of shale.
The list above given is the first corrected catalogue. I believe, yet published of the rich fauna of the Dudley limestone. It might be greatly extended.
CHAPTER VIII.
Description of the Rocks—continued.
Igneous Rocks.
There are two varieties of igneous rock in the district; namely, Basalt and Greenstone, and another apparent variety, differing greatly from both in mere external characters. All three, however, are different parts of the same great mass of molten matter, their variety depending probably more on the conditions under which they have been placed than on any difference in the origin or mineral constitution of the rocks themselves.
The "Rowley rag" is a Basalt, a bard, heavy, black, close-grained rock, weathering brown outside, having a tendency to form spheroids that envelope with several concentric coats a solid ball in the middle, and consequently often assuming a columnar structure, which in some instances becomes nearly as regular as that of the Giant's Causeway. This is the stone of the Rowley Hills. Barrow Hill at Pensnett. Pouk Hill at Bentley, and other spots.
Of this rock an analysis has lately been made by Mr. Henry of London, for Mr. S. H. Blackwell, of Dudley, who has kindly placed it at my disposal.
Composition of Rowley Rag (specific gravity 2.907).
| Silica | 49.860 |
| Alumina | 12.750 |
| Lime | 8.710 |
| Magnesia | 4.395 |
| Protox, iron | 11.380 |
| Perox. iron, with manganese | 3.360 |
| Soda | 5.250 |
| Potash | 0.570 |
| Titanic acid | 1.330 |
| Phosphoric acid | 0.580 |
| Water | 2.560 |
| 100.745 |
Burrowing in the Coal-measures, and here and there coming out to the present surface, is another igneous rock called " green rock" by the colliers. This is generally, if not always, the true "Greenstone" of mineralogists, composed of orthoclase and hornblende,[1]
- ↑ On the authority of Sir H. De la Beche.
