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Suggestions for the study of the chemical geology of the Bathgate Hills

1872; Zoological Society of London; Volume: 2; Issue: 1 Linguagem: Inglês

10.1144/transed.2.1.73

2052-9414

Andrew M. Taylor,

Geological and Geochemical Analysis

1. General Petralogical Characters of the Rock Masses of the District. —The district, bounded by a line from the Shotts to Bo’ness on the west, and thence encircling the intervening country betwixt those points and Edinburgh, is remarkable for the trap beds lying conformable to and interbedded with the aqueous strata. Professor Geikie and Mr Cadell, amongst other authors, have detailed special instances of these phenomena. The area surrounding Bathgate, which has been illustrated in an unpublished section deposited with the Society, also well exhibits this; see also Hay Cunningham’s ‶Geology of Fife and the Lothians,″ plates 9, 12, and 13, exhibiting, amongst others, instances of local chemico-geological action at Hound Point, Winchburgh, and Kirkton. 2. Minuter Petralogical Peculiarities. —In the sedimentary rock masses, as in the sandstones of Kingscavel and in the trap rocks, a globular structure predominates. In the spheroidal masses in the trap rocks may be found lumps of sandstone, coal, and bitumen mixed with calcite. Good specimens of this may be obtained on the road leading from Bathgate to the silver mine near the Raven’s Crag. Many trap rocks emit a strong bituminous odour. Elaterite may be obtained either in particles disseminated through the rock masses or in small veins. The famous Torbanehill mineral, and the bituminous or oil shales, so abound in the district as to form the main sources of its commercial importance. They may be regarded, too, as unique, from the ordinary Coal Measures in their formation. 3. Modern Salses and the Felspathic Ash-beds of the District. —A detailed examination of some district abounding in mud volcanoes is desirable, specially on the mode of eruption of those salses, of their products, and the alterations they effect on the strata and the landscape. Many phenomena, particularly in our coal fields, now handed over to volcanic agency, might be specially referred to these hitherto neglected agents of change. The felspathic ashes, the trappean prevalent species in eastern Linlithgowshire, present mineral characteristics very analogous to those of the high hills now being formed by salses in the Crimea or in Central America; for they, too, contain intercalations of marls, fragments of sandstone, and coal. The intimate relation, in eruptions of the salses in the Crimea and Trinidad, of flows of bitumen, periodically, from subterranean reservoirs, will be remembered in our local study. Iron, in various chemical combinations, flows directly and abundantly from modern mud volcanoes. Might not a large proportion of the iron of the local western Coal Measures have been similarly derived? Mr Binney * insists on the volcanic origin of the ironstones of the Lancashire Coal Measures. The iron, emitted as the sesquioxide, mingled in the waters of the old carboniferous lakes with the roots of plants or other decomposing vegetable matter, which, robbing it of oxygen, changed it into the protoxide. 4. The close Stratigraphical Analogy of the Local Shale Fields and the Pennsylvanian Oil Wells is very striking. Both have been proved to lie stratigraphically beneath the horizon of the true Coal Measures; and both are in the intermediate ground betwixt the Mountain Limestone and the Old Red Sandstone formations. A school of American geologists derive many of their coal-beds from exudations of bitumen from subterranean reservoirs rather than from vegetable origin. This Scottish district is a fair field to test this hypothesis by chemico-geological study. The Silurian rocks of southern Scotland, which ought to be subjacent to the surface rocks, sparsely yield carbonaceous matter. But the mica schists of Inverness-shire, probably even lower in the geologic scale, yield albertite. And a large area of mica schist, thickly impregnated with bitumen, has been recognised in the province of Wenland, in Sweden. The soundings of the North Sea are not such as to preclude the idea that these Swedish rocks might underlie the lower carboniferous series of Fife and the Lothians. 5. The unequal Contraction of the Sedimentary Strata was pointed out by Mr Babbage as a vera causa in physical geology. Professor Philips asserts that a change of temperature of 100° F., in a solid mass 500 miles across, would cause a change of linear dimensions of a quarter of a mile in limestone or sandstone. If the pressure found relief in a vertical fault, it must be sixteen miles down; if in an anticlinal curve, it must be eight miles in height. Mr Adie found a rod of Carara marble to be permanently elongated by the application of heat; but clay is permanently shortened. Limestone beds, marine or fresh-water, associated with trap rocks, constitute the materials of the Bathgate hills. The marine limestone skirts the summit of the hills. The alternate marine, estuarine, and fresh-water fossils evidence frequent physico-geographical changes in the district. These in their turn, too, must have materially affected the changes in strata temperature, consequent on the continuous and wide-spread trap eruptions conterminous with them. How far were those changes influenced by the systematic intercalation of the argillaceous and limestone beds? Were the elongated limestones rent by the simultaneous contraction of the shales and fireclays; and did the salses thus find vents to emit the felspathic ash? In other words, was volcanic actively a consequence rather than a cause of flexure? Professor Hull, in various papers, shows that the occurrence of alternations of argillaceous and calcareous strata follows a regular law. Physical consequences of this arrangement, such as faulting, must also be correlated in a regular manner. Professor Haughton demonstrates the entire system of primary, conjugate, and secondary points occurring in any district to be an easy consequence of the same system of forces acting upon that district; and that it does not necessitate the hypothesis of distinct sets of forces for each system of faults or mountain axis. Such forces would always assume the line of least resistance. A chemico-geological survey, then, must include a study of the faulting as well as the petralogical peculiarities of the district. In the light of the just mentioned researches, it may be found that the opposite systems of faulting traversing it may be referred to one force rather than to separate forces acting in different geological ages. And this must be put in evidence when estimating the general stratigraphical position of the beds. The convoluted strata of the Bathgate hills, whether minutely plicated in hand specimens from the Kirkton quarries, or as seen bending in great waves through the various quarries from Petershill to near Linlithgow, may be adduced as showing the existence of two transverse forces in raising the hills. But the remarks of the late Professor Rogers ought to be borne in mind. Referring to the experiments of Sir James Hall and others, he says—‶It has been alleged that this folding of the clay or cloth is an exact imitation of the flexures of strata seen in nature; but I must deny the assumed analogy. The plications thus produced are merely irregular contortions; they exhibit no definite form of curvature, no constancy in the direction of their gentler and steeper slopes, and no law of regular gradation. Their anticlinal and synclinal axis planes, if they can be said to have any, lean some one way and some another; and the flexures, when the crowding is great, have a tendency to the horse-shoe form, and not to that of waves.″ * 6. Influence of Hot Springs on the Rock Masses. —In areas like that of the Yellowstone and Firehole rivers in North America, where hot springs prevail, mud springs abound; so the observer, while remembering salse action, may look out for tokens of thermal action also, in his survey of the district. Petralogical characteristics of this may be seen in the limestone beds of Kirkton, for whose origin Dr Hibbert invoked this agency; in that of Petershill, whose marble-like appearance betokens strong metamorphism. Indeed, the marine limestones of the hills are unique, first from beds of regularly stratified trap being intercalated with them, both resting on the intrusive boss of the Raven’s Crag, as shown in the MSS. diagram, and from their thinning out a few miles from the hills. It appears, then, impossible to assign their origin simply to organic agencies. The mineral veins cutting through the limestone contain calcite, barytes, argentiferous galena, cobalt, &c., and all these might have been deposited from solution in water. Through the enterprise of Henry Aitken, Esq. of Falkirk, the ancient silver mine of the hills is now being tentatively opened. A vein of argentiferous galena cropping out at the surface was long known; but the following extract from Sir Robert Sibbald’s ‶ Scotia Illustrata, ″ p. 31, shows that this was not the stuff the ancient miners turned out: ‶In Lothiana Occidentali, . . . in monte qui Carnpaple dicitur, tempore Jacobi sexti primi Brittaniæ Monarchæ, ab Alexandro Mund Carbonario inventa est argentifodina, ubi purius argentum, idque majore proportione, ex lapide rubro extractum fuit.″ The pyrargyrite or red silver ore has not yet been discovered in mining, though the possibility of its occurrence is proved by its being found in a barytes veinstone in trap on the Ochils and at Campsie. Mr Aitken has found the course of the vein which the old miners explored much altered by contact with the stratified trap below the limestone; this rock, again, assumes a more felspathic character at the points of junction than through its regular course. A pocket of kupfer-nickel, containing 3 per cent. of cobalt and 37 per cent. of oxide of nickel, excited great hopes, but soon died out. The occurrence of such patches of these and allied minerals may not presage ultimate commercial success: but; along with the fact that the region was the site of ancient gold diggings, these all evidence thermal action; the more so, that such springs are now depositing the precious and other metals in California. Bunsen suggested the derivation of all the varieties of trap from an original magma by subsequent chemical change. How far may the igneous rocks of the district have been altered by hot springs? In truth, may they not be thus now undergoing alteration? Sterry Hunt divides mineral waters, first, into those where silica is in excess, and potash predominates; and, secondly, into those in which the proportion of these elements is reversed. Waters issuing from limestones contain soluble lime and magnesian salts. Now, as both marine limestones and the traps above them contain excess of these chemical compounds, hornblende and pyroxene will abound in those superincumbent rocks. In waters issuing from argillaceous strata, on the contrary, carbonate of soda predominates, besides lime magnesia, and occasionally iron; the rocks through which these percolate will abound rather in felspathic minerals. What metamorphic effects have existing mineral springs? Professor Fleming used to drink an effervescing water from a spring near Blackburn, in the house of Mr Thom, the eminent hydraulic engineer. Report gives peculiar virtues to many mineral springs in West Lothian. An investigation of them, specially in their chemico-geological relations, would well repay the chemist.