Fig. 227.—Vein of "white-trap" cutting black carbonaceous shales, a little west from St. Monans Church.
That these great intrusions took place later than the deposition of the Coal-measures is obvious. There is no satisfactory evidence to enable us to decide to which of the two post-Carboniferous volcanic periods they may with most probability be assigned. As one of them is distinctly cut by dykes that have been referred to the Tertiary series, it might be plausibly argued that it at least is of pre-Tertiary date, and therefore probably Permian. On the other hand, as will be shown in a later chapter, some portion of the sills appears to be connected with the younger or Tertiary dykes. This problem must for the present remain unsolved.
In Ayrshire where, as already described, basic sills traverse the Permian volcanic series, other large intrusive sheets are found around the Permian basin. On the north side an important group of them, passing through the Coal-measures into the Carboniferous Limestone series, runs from Troon eastward for more than eight miles to beyond Craigie. On the south side a much more extensive series may be traced from the River Ayr southwards into the Dalmellington coal-field, and thence north-eastwards in a wide semicircular sweep into the coal-field of New Cumnock and Airds Moss. That some of these sills proceed from the Permian necks has been definitely ascertained, and this fact has been already alluded to in connection with the vents. I have little doubt that the great majority, if not the whole, of these intrusive sheets are to be referred to the Permian period.
Some of the sills must be later than a part of the Permian volcanic eruptions, for they are found in at least three places intercalated in the zone of lavas and tuffs. But no instance has been observed of their traversing the basin of Permian sandstone which overlies that zone, though a few dykes, possibly of Tertiary age, do cut this sandstone.
CHAPTER XXXII
PERMIAN VOLCANOES OF ENGLAND
The Devonshire Centre—Eruptive Rocks of the Midland Coal-fields.
From the south of Scotland we need to pass to the extreme south-west of England before we again encounter a group of volcanic rocks which may be referred with some confidence to the Permian period. An interesting group of lavas and tuffs has been preserved in some of the valleys over a limited area in the east of Devonshire. The Midland coal-fields, however, are traversed by a series of basic eruptive rocks which are younger than the Coal-measures, and may possibly be Permian. Their mode of occurrence, and the arguments regarding their geological age, will be given in the present chapter.
1. DEVONSHIRE
The counties of Devon and Cornwall furnish one of the most striking examples to be met with in Britain of the persistence of volcanic action over a limited area through a long succession of geological periods. The extensive eruptions in Devonian time were followed after a long interval by a diminished series in the Carboniferous period. But the subterranean energy was not then wholly exhausted, for it showed itself on a feeble scale in at least one limited tract of the same region during the Permian period. Thus throughout the later half of Palæozoic time the extreme south-west of England continued to be a theatre of volcanic action.
The geological age of the igneous rocks now to be referred to depends upon the particular place in the geological record to which we assign the remarkable breccias and sandstones with which they are associated. By many geologists who have been unable to recognize any true break in the red rocks from their base up to the bottom of the Lias, these strata have been grouped as one great series referable to the "New Red Sandstone" or Trias. This is the classification adopted on the one-inch maps of the Geological Survey. On the other hand, various able observers have pointed out the close resemblance of the coarse and fine breccias at the bottom of the series to recognized Permian deposits in the centre of England and to parts of the typical Rothliegende of Germany. I need only refer to the strongly expressed views of Murchison, in which, as he stated in his Siluria, he "entirely agreed with Conybeare and Buckland, who, after a journey in Germany in 1816, distinctly identified the Heavytree conglomerate, near Exeter, with the Rothliegende of the Germans."[104] In the absence of any fossil evidence, we have only lithological characters and sequence to guide us, and though the known facts hardly warrant a very positive opinion, my inclination is to regard these red Devonshire breccias as probably Permian, and to follow Murchison in looking upon their associated igneous masses as furnishing additional reason for assigning them to that particular geological platform.[105]