lavas, while the interval of quiescence between them is represented by a thousand feet of limestone. The same observers likewise mapped outside the volcanic ring a number of eruptive bosses, which they regarded as probably marking some of the actual vents of that time. [Footnote 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.] [Footnote 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. [Footnote 74: Explanation of Sheet 144, p. 27.] [Footnote 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.]