But Croghan Hill, though the chief, is not the only vent of the district. It forms the centre round which a group of subsidiary vents has been opened. These form smaller and lower eminences, the most distant being one and a half miles E.S.E. from the summit of Croghan Hill, and measuring approximately 1200 feet in its longest and 800 feet in its shortest diameter.
That the igneous materials of these necks really break through the limestones may be clearly seen in several sections. Thus by the roadside at Gorteen, on the south-western side of Croghan Hill, the limestones have been thrown into a highly inclined position, dipping towards the east at 60° or more, and their truncated ends abut against the side of the neck. Again, on the eastern side of the same hill the limestones have been much disturbed close to the margin of the neck, sometimes dipping towards the volcanic centre, and sometimes striking at it. Among these strata a small neck of breccia, of which only a few square yards are visible, rises close to the edge of the bog that covers the adjacent part of the great plain.
The material which chiefly forms these necks is one of the most remarkable breccias anywhere to be found in the volcanic records of the British Isles. The first feature noticeable in it is the pumiceous character of its component fragments. These consist of a pale bluish-grey basic pumice, and are generally about the size of a hazel-nut, but descend to mere microscopic dust, while sometimes exceeding a foot in length. They are angular, subangular and rounded. Occasionally they stand out as hollow shells on weathered surfaces, and in one instance I noted that the vesicles were flattened and drawn out parallel to the surfaces of the shell, as if deformed by gyration, like a true bomb.
The breccia remains singularly uniform in character throughout all the necks. Its basic pumice presents much resemblance to that so characteristic of the Carboniferous necks of Scotland, Derbyshire and the Isle of Man. The abundant vesicles are generally spherical, and as they have been filled with calcite or chlorite, they look like small seeds scattered through a grey paste. Though I broke hundreds of the lapilli, I did not notice among them any volcanic rock other than this pumice. I am not aware of any other neck so homogeneously filled up with one type of pyroclastic material, and certainly there is no other example known in the British Isles of so large and uniform a mass of fragmentary pumice.
Limestone fragments are not uncommon in this breccia. They resemble the strata around the vents. Pieces of the adjacent cherts may also be observed. In one or two cases, the limestone fragments were found by me to have an exceptionally crystalline texture, which may possibly indicate a certain degree of marmarosis, but on the whole there is little trace of alteration.
The fragments of pumice in the breccia are bound together by a cement of calcite. In fact the rock is, so to speak, saturated with calcareous material, which, besides filling up the interstices between the lapilli, has permeated the pumice and filled up such of its vesicles as are not occupied by some chloritic infiltration.
I did not observe unmistakable evidence that any part of the breccia is stratified and intercalated among the limestones, nor any vestige of ashy material in these limestones. But it is possible that traces of such interstratification may occur in the low ground to the north-west of Croghan Hill, which I did not examine.
In only two places did I notice even a semblance of the intercalation of limestone in the breccia. One of these is at Gorteen, where a band of limestone strata a few feet thick is underlain and overlain by breccia. But though the superposition of the layers of finely stratified dark limestone and chert on the breccia is well seen and thoroughly defined, no lapilli or ashy material are to be seen in the limestone. Detached pieces of similar limestone and chert occur in the breccia. The band of stratified rock, if in situ, may be a tongue projecting from the wall into the body of the neck, like some instances already cited from Scotland, but more probably it is really a large included mass lying within the vent itself. The breccia here as elsewhere is entirely without any trace of stratification. The second locality occurs at the most easterly neck north of Coole House, where the limestones, rapidly undulating, seem at last to plunge below the breccia, which shows a series of parallel divisional planes suggestive of bedding. But these may be only joint-structures, for there is no stratification of the component materials of the rock.
In the necks, and also through the limestone surrounding them, masses of eruptive rock have been intruded as irregular bosses and veins. The material of these intrusions presents little variety, and, so far as I could note, gives no indication of the successive protrusion of progressively different lava. It varies from a deep blue-black fine-grained basalt to a dolerite where the plagioclase is distinct. Some portions, however, are more basic and pass into limburgite. Externally there is nothing worthy of special remark in these rocks unless it be their prevalent amygdaloidal structure. The amygdales, generally of calcite, vary from small pea-like forms in the basalts up to kernels half an inch long or more in the dolerites. From a microscopic examination Mr. Watts found that some of the basalts have a base of felspar and augite rich in brown mica, and that their porphyritic felspars enclose idiomorphic crystals of augite.
Perhaps the most noticeable feature in these later parts of the volcanic series is the occurrence in them at one locality in Croghan Demesne of lumps of a highly crystalline material quite distinct from the surrounding rock. These enclosures vary from an inch or two to a foot or more in diameter. They must be regarded as blocks which have been carried up in the ascent of the basic lava. Their composition has been ascertained by Mr. Watts from microscopic examination to be somewhat singular. One specimen "contains relics of garnets, surrounded by rings of kelyphite, imbedded in a mosaic of felspar, with a mineral which may possibly be idocrase." Another specimen from the same locality (south-east from Gorteen) "contains the relics of garnets preserved as kelyphite, set in a matrix of quartz-grains, much strained, and containing a profusion of crystals of greenish-yellow or red sillimanite. This appears to be a metamorphic rock, and may be a fragment of some sediment enclosed in the igneous rocks."[71]