*450 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
several bands of igneous rock wliich occur here is made quite evident by the 
alteration they have produced upon the shales with which they have come 
in contact. It is the uppermost and most extensive of tliese sills whicli 
specially deserves notice, for the differentiation of its constituents. It 
stretches along the cutting for several hundred yards at ari angle of dip of 
about 15°. At the western or upper part of the mass its actual contact 
with the superincumbent sedimentary strata is not visible, but as the 
igneous rock is there a good deal finer in grain than elsewhere, its upper 
surface cannot be many feet distant. The upper visible portion is a light 
161. — Section of Sill, (Jraniond Kaihvay, Barutoii, near Edinburgh. 
1. Raked sliale ; 2. Sill of very felspatliic dolm-lte about nine feet thick ; 3. Raked shale, ci^ht inches : 4. Dolerite 
sl)Owin<^ chilled line-graincjl etlge and ailhering llriiily to the shale below; it rapidly passes up into ( 5 ) Picrite 
with wliite felspathic veins (0); 7. Junction of picrite and dolerite* with a similar vein along the line of 
contact ; S. I./£irgo globular body of dolerite enclosing a mass of picrite. 
well-crystallized dolerite with a rudely bedded structure, the planes dipping 
westwards at 15°. About 20 or 30 feet below the upper visible termination 
of the mass, the dark ferro-magiiesian minerals begin rapidly to increase in 
relative proportion to the pale felspar, and the rock consequently becomes 
dark-greenish brown. The change is particularly noticeable in certain 
bands which run parallel witli the general dip. There is no definite line 
between the pale and dark body of the rock, the two graduating into each 
other and the darker part becoming deeper in colour, heavier and more 
decomposing, until it becomes a true typical picrite. Even in this ultra-basic 
portion the same rude bedding or banding may be observed. 
\eins in which felspar predominates over the darker minerals traverse 
the rock, sometimes parallel with the bedding, sometimes across it. They 
vary from less than an inch to a foot in width, sometimes dividing and 
enclosing parts of the surrounding mass. But that they are on the whole 
contemporaneous with the sill itself, and not long subsequent injections, is 
sliovvn by the way in which the dark ferro-niagnesian minerals project from 
the picrite into the veins and lock the two together. 
But besides these injections, which doubtless represent the last and 
more acid portions of tlie magma injected into the basic parts before the 
final consolidation of the whole, there are to be observed irregular concre- 
tionary patches, of similar character to the veins, distributed through the 
picrite. On the other hand, towards its base the sill becomes a coarse 
dolerite round which the picrite is wrapjied, and which encloses a detached 
portion of that rock. 
It is deserving of note that while the ultra -liasic portion descends 
almost to the very bottom of the sill, the lowest five feet show the same 
« 
p. 39. Mr. Goodcliild recognized the occurrence of picrite, and Mr. Monckton has described tli® 
succession of rocks, and given a diagram of tliem. 
