4o6 
THE CARBONIFEROUS VOLCANOES 
KOOK VI 
by a great hollow, measuring about a mile in length and half a mile in 
breadth (Fig. 128).^ It is occupied mainly by a coarse tumultuous agglo- 
merate, like that of other necks in the same district, but with a matrix 
rather more indurated, and assuming in certain parts a crystalline texture, 
so as to be at first sight hardly distinguishable from some of the surrounding 
andesites. Even in this altered condition, however, its included fragments 
may be recognized, particularly blocks of sandstone which have been 
hardened into quartzite. ISTumerous small veins of pink and yellow trachyte 
traverse the agglomerate, and are found also cutting the bedded andesites 
that encircle it. 
8. DYKES AND SILLS 
Intrusive masses both in the form of dykes and of sills are of frequent 
occurrence in connection with the Carboniferous volcanic plateaux. From 
the variety of their component materials it may be inferred that these rocks 
belong to different ages of intrusion. 
Dykes. — The great majority of the Dykes consist of trachyte or of 
andesite, resembling in lithological characters the material of the necks and 
doubtless connected with its uprise. There occur also dykes of diabase, 
basalt or dolerite. Some of the latter, especially those which run for many 
miles, cutting every rock in the districts in which they occur, and crossing 
large faults without deviation, are certainly long posterior to the pilateau 
volcanic period. Whether the small inconstant dykes of more basic com- 
position, found in the same districts with the trachytes, are to he looked 
upon as part of the volcanic phenomena of the plateaux, is a question to 
which at present no definite answer can be given. I shall have occasion to 
show that in the next volcanic period the lavas that flowed from the puys 
are more basic than most of those of the plateaux, and that they are 
associated with more basic dykes and sills. In Ifoxburghshire, where it is 
so difficult to distinguish between the denuded vents of the two periods, 
the dai’k heavy olivine-basalts and dolerites of the bosses may possibly 
belong rather to the later than to the earlier volcanic episode. And if 
that be their true age, the dykes of similar material may be connected 
with them. At the same time it must be remembered that the earliest 
eruptions of the plateaux were markedly basic, that many vents in the 
plateaux are pierced by basic intrusions, and that basic dykes may have been 
associated with the uprise of the same magma. 
Tlie dykes occur in considerable numbers and in two distinct positions, 
though these may be closely related to each other: 1st, among the rocks 
outside and beneath the plateau-lavas, or cutting these lavas ; and 2nd, in 
and around the vents. 
1. Among the rocks which emerge from under the Carboniferous 
volcanic plateaux, dykes are sometimes to be observed in considerable 
numbers. They may he compared to the far more extensive series con- 
’ See Exiilanatioii to Sheet 31, Geological Stmey of Scotland, par. 21 (1878). 
