CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 515 
usually empty in the preparations; but calcite occasionally remains in 
them 
Besides the abundant cells, this substance frequently contains prisms of a 
triclinic felspar, and sometimes slender needles, which may be apatite. I have 
also observed round and subangular granules of quartz, containing abundant 
liquid cavities. This quartz is probably an accidental constituent caught up 
in the original melted rock, and not properly belonging to its composition. 
Scattered rod-like and granular black opaque microlites are sometimes 
observable. Very minute black grains may likewise be noticed, more par- 
ticularly round the circumference of the cells.* 
Viewed with ordinary light this green or yellowish transparent glass-like 
base of the lapilli at once suggests Palagonite. Between crossed Nicol prisms it 
is resolved into the pale bluish grey or neutral-tinted finely fibrous appearance, 
with occasional bright chromatic polarisation so characteristic of serpentine. 
There can be no doubt that this serpentine or serpentinous substance must 
have been originally a glass, in the most thoroughly melted condition, and that 
it was kept in brisk ebullition by the passage of vapour through it. It has 
no counterpart among the lavas erupted at the surface. The nearest analogy 
is to be found in the Blackburn ‘‘pikrite” already described; but there is 
nothing in that rock like the minutely and abundantly cellular structure of 
the lapilli in the tuffs. These fragments occur chiefly in the tuff of vents ; 
they abound in the necks of the Fife coast, sometimes to such an extent as 
nearly to constitute the entire mass of the tuff, as at Kilmundy Hill, near 
Burntisland. They may be observed in the later agglomerates of Arthur Seat, 
at St Magdalene’s, Linlithgow, &c. They occur less frequently in the inter- 
stratified sheets of tuff, as among some of the beds at Pettycur and Kinghorn. 
I regard them as having been derived from the explosion of a rock which con- 
tained little felspar, but probably consisted mainly of olivine and augite, and 
which remained for a long while simmering, as it were, at the bottom of some 
of the volcanic vents. I have pointed out, in the case of the remarkable 
Blackburn rock, that a segregation of its materials had taken place, the heavy 
olivine remaining chiefly below. Something of the same kind may be supposed 
to have occurred in the volcanic funnels. After long fusion the lighter minerals, 
notably the felspar, may have come chiefly to the top, whence they might be 
discharged at successive volcanic eructations. Eventually the lower and 
* Since the above description was written, I have had an opportunity of examining the artificially 
fused products of some of the basalts and dolerites from the neighbourhood of Edinburgh (ante, p. 498, note). 
The resemblance of this altered serpentinous cellular substance, so abundant in some of the volcanic 
vents, to the glass obtained by fusing such a basalt as that of the Lion’s Haunch, is so remarkable as at 
once to suggest an original similarity of condition. This glass, artificially obtained from some rocks like 
that of the Lion’s Haunch, where the felspar resists fusion, must consist mainly of olivine and augite 
with diffused magnetic iron, and, as I have already said, it contains tufted microlites not unlike those 
of the tuff-lapilli. 
