Page:The Geologist, volume 5.djvu/63

Rh be only explicable on the hypothesis that they were emptied at the same time that the other strata were deposited.

The chemical changes effected by these igneous strata on the surrounding rocks are likewise very curious. In many places the limestone is changed into a crystalline marble. One bed at Kirkton affords undoubted evidence that it was deposited by a thermal spring. The great thickness the main bed of limestone in the hills attains, may be accounted for as much from its being a chemical deposit, as one of animal origin. The sandstones and shales, too, are often curiously baked, showing the violence of the igneous agencies. But we call special attention to the prevalence of bitumen in the district, sometimes appearing solid in the crevices of the sandstones, as at Binny; sometimes in round circular nodules in the trap or limestones; and sometimes oozing out liquid from trappean reservoirs.

The circular type of structure is very prevalent in the aqueous rocks of the district, as in the sandstone at King's Cavel, and amongst the ironstones. It extends throughout the rock systems. It is most manifest in the oolite or roe-stone of another formation. However we may explain it, it is clearly the result of agencies at work when the sandstones and shales were depositing, and not a subsequent chemical change. This admitted, it follows that most of the bitumen of the district is contemporaneous with the igneous rocks, and that the highly bituminous sandstones and shales were saturated at the period of their deposition. The clearest proof of this is the structure of the celebrated Binny sandstone. How else can we explain the black bituminous patches appearing on its surface? The physical agency at work may have been the conjunction of two rapid currents. But it is much easier to suppose the bitumen ejected from some neighbouring volcano floated in the waters of the lagoon or river in which the sandstone was forming, and then mechanically mingled with it; than that the sandstone was subsequently saturated from beneath.

Facts connected with the occurrence and formation of bitumen at the present day bear out this hypothesis. Its connection with volcanic agency is well known. The celebrated pitch-lake of Trinidad stands in close proximity to a volcano, as also do some of the bituminous localities in Asia Minor. All the three varieties of this substance float on water. Maltha, or mineral pitch, floats on the surface of the Dead Sea. Petroleum floats on the Tigris and Euphrates, so much so, that the surface of the river is often set on fire. The boatmen on the Tigris and Euphrates are paid in this substance. Doubtless at the bottom of these rivers there are many nascent beds of richly bituminous shales.

Given then a series of submarine volcanos ejecting out sheets of liquid bitumen, and at the same time sand and mud rapidly deposited; let these commingle, and we have the rationale of the formation of the Binny sandstone, and the bituminous shales of Queensferry and Broxburn. These forces ceased after a time. A morass was slowly formed which now constitutes the Houston coal-bed. This indicates