Page:The Geologist, volume 5.djvu/64

46 another condition under which bitumen w as eliminated. In this case it is the result not of mechanical deposition, but of subsequent chemical action from decaying organic substances. Again, the action of the currents was resumed, and fresh bituminous shales were formed.

When the contemporaneous traps on the north-west side of our section were ejected, the same succession of physical changes continued. Bitumen occurs in globules both in the contemporaneous traps and in the limestones. The limestones indicate three marked alterations in the level of the land. First, the Kirkton limestone, with its leafy laminæ, and curiously baked beds of cherty porcelain, its interstratified ash, and over-capping basalt indicate proximate volcanic activity when forming. Fluvio-marine fossils are found in it. The land then sank so far as to allow the building corals to commence their labours; a reef was now formed which was added to by shells dashed in by the surf from the neighbouring sea, and the precipitation of carbonate of lime from a sea surcharged from its proximity to a volcanic cone; thus the great belt of the limestone of the hills was formed. But immediately after the land was subject to as rapid an elevation; as is manifest from the Stigmarias found in the upper bed of the limestone,—the lower beds abounding in deep-sea shells. Ash-beds also cover it. The hills now seem gradually to have risen above the waves, and a prevalence of freshwater strata filled the small Torbane Hill basin. But all this time the volcano did not stop its activity, as is evidenced by the thick ridges of interbedded basalt which may be traced terracing the country upwards from Bathgate. It is easy to suppose that sheets of bitumen, as at the prior period of the Binny sandstone, floated on the waters of this lagoon; that in one time in particular, a very large quantity was given out, and thus, aided mayhap by ejections from beneath, the Torbane Hill bed was formed. May not the round circular masses in the Torbane Hill mineral, which so puzzle microscopists, be the result of the action of currents,—only, however, on a smaller scale than those visible to the naked eye in the other rocks of the district? In suggesting this hypothesis we make allowance for the fact that at other times the basin was elevated so that morasses could accumulate, and thus the beds of coal be formed. The district thus exhibits evidence of both modes of the elimination of bitumen.

In the upland country west of the Torbane Hill basin there is a singular absence of trappean ridges. The district rises into a series of undulating hills formed solely of the upper members of the carboniferous system of Lanarkshire. The lower carboniferous volcano had ceased previous to their deposition; and the Bathgate hills probably formed elevated land at the base of the great strait in which these strata were depositing. Slowly the land rose and fell, morass after morass accumulated to be compressed into future coal-beds after being covered over by sand and mud. Bitumen was thus formed through chemical agencies. Its source is manifest from the microscopic structure of the coal, which is entirely of woody origin, not exhibiting traces of clay or sand from drift. The beds of this upper formation