480 PROFESSOR GEIKIE ON THE 
which they overspread unconformably. As a provisional arrangement, I would 
class them as probably of Permian date. That some, at least, are older than 
the Tertiary volcanic period is established by the fact, pointed out by me many 
years ago, that they are traversed by the great series of east-and-west Miocene 
dykes, though others in Stirlingshire may possibly be parts of an outflow from 
these dykes. For the reason, however, above given in regard to including the 
younger volcanic necks of Fife in this paper, I do not exclude these latest sheets. 
Dykes.—Excluding the east-and-west Miocene dykes, there are comparatively 
few dykes in the Basin of the Forth save in connection with the volcanic necks, 
as already described. Among the red sandstones on which the city of Edin- 
burgh is built, a number of dykes have been cut in draining, well-sinking, and 
other operations. Among the streams of the same neighbourhood an occa- 
sional dyke or vein may be seen traversing and involving the shales or sand- — 
stones. But the contact-phenomena do not call for any special remark in 
addition to what has been already said in reference to the intrusive sheets. 
Some of the curious features of internal structure presented by the dykes 
will be described in the Second Part of this Paper. 
3. Bedded Lavas and Tuffs. 
The minor cones in the region discharged in most cases only showers of tuff. 
Hence it is common to meet with beds of stratified tuff intercalated among the 
ordinary Carboniferous strata, without any other volcanic accompaniment. 
Numerous examples occur in Linlithgowshire and the western part of Edin-— 
burghshire, as well as in Fife. The tuff of these solitary bedsis seldom coarse- 
grained. It usually consists of a fine dirty-green granular debris, derived from 
the trituration of the pyroxenic lavas of the period, and mixed with fragments 
of sandstone, shale, limestone, and other stratified rocks. It is often seamed 
with layers of ordinary sedimentary matter, probably indicating that its eruption 
did not occur at once, but was prolonged, with occasional pauses. The 
gradation of the upper part of the tuff into the overlying strata is often 
insensible, shewing that while the showers of tuff grew feebler, the usual sedi- 
ment of the lagoon imperceptibly regained possession of the bottom. 
From some of the larger or at least more close-set vents tuff continued to 
be thrown out for a long period, without the appearance of lava. The thick 
tuffs of Dunbar and North Berwick mark the earliest eruptions of the East 
Lothian district. The Saline vents threw out only tuff to a depth of several 
hundred feet. The numerous vents in the east of Fife likewise produced 
chiefly tuff, although lava certainly rose in many of them, if it did not actually 
escape at the surface in those wide sheets already noticed. 
Lava-cones, answering to the solitary tuff-cones, do not appear to have 
existed, The lavas never occur without tuffs, except here and there where a 
il 
4 
