CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 513 
and length of the eruption and to the proximity of the vent will be the thick- 
ness of the layer of tuff and its freedom from extraneous materials. But as we 
recede from the centre of disturbance we find the volcanic debris to be more and 
more commingled with ordinary sediment until at last it comes to be no longer 
traceable in the usual sand or silt of the district. Hence, in dealing with the 
bedded tuffs of the Carboniferous system in the region of the Forth we are 
constantly presented with varying mixtures of fine volcanic debris and ordinary 
mechanical sediment. The reality and nature of this commingling can best be 
seen when the non-volcanic material is limestone, as may be instructively 
observed among the Kirkton Quarries to the east of Bathgate, and on the Fife 
coast between Pettycur and Kirkcaldy. (See ante, p. 482.) 
The bedded tuffs vary according to the nature of the lavas with which they 
are associated. In the porphyrite districts they are dull red or greenish rocks, 
made up of fine porphyrite debris, mixed with ordinary sand and clay. In the 
doleritic and basaltic region they are almost invariably of a characteristic 
blackish-green to sage-green tint, rarely dull yellow or red; and are well strati- 
fied, the layers being marked off by lines of lapilli, consisting of greenish decayed 
varieties of basalt rocks. Their layers vary from mere lamin, scarcely thicker 
than writing-paper, up to thick beds piled over each other to a depth of several 
hundred feet. Organic remains are frequently to be met with in these tufts. 
Thus at the east quarry, Kirkton, we may observe well-preserved fronds of 
Sphenopteris and Pecopteris, with stems of Lepidodendron and Calamites ; at the 
west quarry, Productus longispinus, crinoid stems, and other marine organisms ; 
at St Anthony’s Chapel, Arthur Seat, scales of Rhizodus, and other fish 
remains. 
2. Microscopic Characters.—The fragmental rocks do not yield such satis- 
factory results as the crystalline masses to investigation with the microscope. 
In thin slices, with a low magnifying power, they are seen to present the same 
twofold composition as on the large scale to the naked eye, viz., enclosing 
paste and enclosed fragments. 
a, The Paste.—I have never yet succeeded in obtaining any definite structure 
in the matrix of the tuffs. It is a dull, finely granular amorphous substance, 
which under a high power is resolvable into shapeless grains and shreds, 
often greenish, sometimes colourless, and sometimes black and opaque. There 
can be no doubt that these particles are merely the more thoroughly com- 
minuted debris of the same materials as constitute the distinct lapilli. In 
no case have I found any microlites such as are met with in some modern 
volcanic tuffs and ashes. If any such ever existed, they have disappeared in 
the general oxidation and alteration of the matrix of the rock. The tufts, 
being commonly porous incoherent masses, have suffered more from the 
influence of percolating water than the solid basalts. Probably we never see 
VOL. XXIX. PART I. 6 Q 
