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576 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
One of these (A) shows the ordinary composition of the coal at a distance 
from the influence of the intrusive rock, the second (B) taken from a 
distance of about 0°3 metres (nearly 1 foot) exhibits a partial conversion 
into coke, while in the third (C), taken from immediate contact with the 
eruptive mass, nearly all the volatile hydrocarbons have been expelled. 
Ash, Sulphur. Coke. Bitumen. 
A. 8:29 per cent. 2°074 79% 20°3 
ee EB “ 1 AEE | Wy 87°8 12°2 
C. 45°96 = 0°151 95°3 4°7 
In a coal-field much invaded by igneous rocks the seams of coal are 
usually found to have suffered more than the other strata, not merely 
because they are specially liable to alteration from the proximity of heated 
surfaces, but because they have presented lines of more easy escape for 
the igneous matter pressed from below. The molten rock has very 
generally been injected along the coal-seams; sometimes taking the 
lower, sometimes the upper surface, or even, as already stated, forcing its 
way along the centre. 
During the subterranean distillation arising from the destruction or 
alteration of coal and bituminous shales, while the gases evolved find their 
way to the surface, the liquid products, on the other hand, are apt to 
collect in fissures and cavities. In central Scotland, where the coal-fields 
have been so abundantly pierced by igneous masses, petroleum and 
asphaltum are of frequent occurrence, sometimes in chinks and veins of 
sandstones and other sedimentary strata, sometimes in the cavities of the 
igneous rocks themselves. In West Lothian intrusive sheets, traversing a 
group of strata containing seams of coal and oil-shale, have a distinctly 
bituminous odour when freshly broken, and little globules of petroleum 
may be detected in their cavities. In the same district the joints and 
fissures of a massive sandstone are filled with solid brown asphalt 
which the quarrymen manufacture into candles. 
Striking as is the change produced by the intrusion of basalt into 
coals and bituminous shales, it is hardly more conspicuous than the altera- 
tion effected on the invading rock. A compact crystalline black heavy 
basalt or dolerite, when it sends sheets and veins into a coal or highly 
carbonaceous shale, becomes yellow or white, earthy, and friable, loses 
weight, ceases to have any apparent crystalline texture, and, in short, 
passes into what would at first unhesitatingly be pronounced to be mere 
clay. It is only when the distinctly intrusive character of this substance 
is recognized in the veins and fingers which it sends out, and in its 
own irregular course in the altered coal, that its true nature is made 
evident. Microscopical examination shows that this ‘‘ white-rock” or 
‘‘white-trap” is merely an altered form of some diabasic or basaltic 
rock, wherein the felspar crystals, though much decayed, can yet be 
traced, the augite, olivine, and magnetite being more or less completely 
changed into a mere pulverulent earthy substance. A specimen of this 
altered rock analysed by Henry gave :—Alumina, 13°250; Silica, 38°830 ; 
Lime, 3°925; Magnesia, 4°180; Soda, 0°971; Potash, 0°422; Protoxide of 
iron, 13°830; Peroxide of iron, 4335; Carbonic acid, 9°320; Water, 
11:010=100°073. It is evident that part of the lime, magnesia, and 
alkalies, and some of the silica, have here been removed, and that most of 
the iron exists as ferrous carbonate. 
