[72] This is the thickness given in the Explanation to Sheet 144 of the Geological Survey of Ireland, p. 8. A still greater thickness is claimed in Explanation to Sheet 154, p. 8.
[73] See especially Explanations of Sheets 143, 144, 153 and 154, Geol. Surv. Ireland (1860, 1861). The geology of the district had been previously noticed by earlier observers, to whose writings reference is made on p. 26 of the Explanation of Sheet 144. See also Jas. Apjohn, Journ. Geol. Soc. Dublin, vol. i. (1832), p. 24; Prof. Hull, Geol. Mag. for 1874, p. 205. Jukes (Student's Manual of Geology, 2nd edit. 1862, p. 325) gave subsequently an excellent epitome of the volcanic history. The microscopic structure of some of the Limerick volcanic rocks has been described by Mr. Allport, Quart. Journ. Geol. Soc. vol. xxx. (1874), p. 552, and by Prof. Hull, Geol. Mag. for 1873, p. 153. See also Mr. Watts' account of these rocks in the Guide to the Collections of Rocks and Fossils (Dublin, 1895), p. 93.
The lower volcanic group, which forms a complete ring round the Upper Limestones of the Limerick basin, is estimated to reach a thickness of 1000 feet in some parts of its course.[74] Its base appears to coincide generally with the upward termination of the Lower Limestone group of this district, though here and there small patches of volcanic rocks in that group have been regarded as interstratified and contemporaneous bands.[75] It consists of a series of lavas and tuffs, the alternations and rapid incoming and dying out of which were well made out by the Geological Survey.
[74] Explanation of Sheet 144, p. 27.
[75] Some of them, however, have characters that rather seem to place them with the intrusive materials of the district, and therefore not necessarily earlier than the bedded lavas and tuffs. The boundary line of the volcanic series is not consistently followed along the same horizon on the Survey maps. Thus to the east of Caherconlish, a strip of the Upper Limestone is inserted below the base of the tuffs for a distance of about four miles. Unless a different horizon has been in some places taken for the boundary between the two groups of limestones, it would appear that the eruptions had not extended over the north and north-east of the district until some time after the deposition of the Upper Limestone had begun. The division between the two limestone groups is taken at a set of chert-bands, but as these are not constant it is sometimes difficult to draw a satisfactory line of division.
Tuffs.—The base of the volcanic series is generally formed by a band of tuff sometimes as much as 350 feet thick,[76] which may be traced nearly continuously round the basin as well as in detached outliers even as far as Carrigogunnel overlooking the alluvial plain of the Shannon. The manner in which the bottom of this tuff is interstratified with the limestone below it may be instructively examined in many quarries around the town of Limerick. Striking evidence is there supplied that the first eruptions were comparatively feeble and spasmodic, and were separated by intervals of longer and shorter duration, during which the limestone with its fragmentary organisms was deposited, little or no volcanic detritus falling at that time. Yet even in some of the limestones the microscope reveals fine broken needles of felspar, representing doubtless the finest ejected dust.[77]
[76] Explanation of Sheet 154, p. 21.
[77] For the details of the microscopic structure of the Limerick volcanic rocks I am mainly indebted to the examination of them made for me by my Survey colleague, Mr. W. W. Watts.
As an illustration of the way in which the volcanic and organic detritus alternated over the sea-floor, the following section from a quarry in the townland of Loch Gur on the southern side of the basin is here given:[78]—
[78] Explanation of Sheet 154, pp. 21, 22.