214 SIR ROBERT CHRISTISON, BART., ON FOSSIL TREES 
stantially correct, and only in so far faulty as the carbonaceous matter of the 
substance of the fossil is called coal. | 
From many analyses, some quantitative, others qualitive only, I have come 
to the following conclusions :—1. That the thin black outer crust is caking coal, 
sometimes very pure, but in some united with the material which fossilises the 
interior; 2. That the whole vast interior is magnesian limestone, strongly 
charged with carbonate of protoxide of iron, and to a less amount with | 
charcoal, or,—so to speak, charcoaly ferruginous dolomite; 3. That there is 
extremely little silica or alumina, and sometimes none, so that these ingre- 
dients are probably adventitious; 4. That the proportion of each ingredient 
varies in different parts of the same fossil, and consequently may seem to differ 
in the several fossils; but 5. That the composition of the interior of all the 
Craigleith araucarious fossils is substantially the same. 
The coal-like crust of these fossils is everywhere in immediate contact with 
their woody structure, of the surface of which it bears the impression. The 
attachment is so slight that the crust is very easily removed. I have examined 
with greatest care the crust of No. 5, which is now in the British Museum, 
Its colour is jet-black, its lustre glassy, its fracture conchoidal, and its brittle- 
ness great. Its density is 1:°225. Acids and alkalis do not act on it. When 
heated in small fragments or in powder in a platinum crucible, it gives out 
much black smoke or dense white flame, froths up and cakes, becomes a 
roundish mass of very light, vesicular, firm, glittermg coke, and when then 
broken up may be burnt away with the exception of a scanty, greyish-red ash. 
The proportion of volatilisable bituminous matter, charcoal, and ash, varies a 
little. The very thin part of the fossil gave 66°8 charcoal, 29-7 bitumen, and 3°4 
ash; but the ash sometimes amounted to 60. A mass, from the lowest part of 
the fossil, nearly as large as the fist, gave 74-0 charcoal, 24:2 volatilisable 
matter, and only 11 to 1°7 ash. Abstracting the ash, the proportion of the 
charcoal to the bituminous matter seems to vary very little. The ash yields no 
alumina to strong potash-solution aided by heat, and only a little lime, but 
neither magnesia nor iron, to diluted nitric or hydrochloric acid; in this respect 
differing entirely from the mineral matter of the great fossil which the coal-like 
covering encrusts. It is evident from this account that the crust presents all 
the properties of the finest caking splint coal. 
The black crust is not in the case of all the fossils so fine a bituminous coal 
as that now described. What is still left attached to Mr WirHam’s fossil, No. 
2, after forty-three years of exposure to the open air, froths up very little when 
heated, cakes but slightly, and leaves when incinerated a large amount of ash, 
apparently the same earthy matter which mineralises the interior of the 
fossils. ’ 
It may be here observed in passing, that the coal-like crust, which is well 

