_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] [Footnote 76: Explanation of Sheet 154, p. 21.] [Footnote 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]-- [Footnote 78: Explanation of Sheet 154, pp. 21, 22.] Cherty limestone more than 20 feet 0 in. Decomposed green tuff 2 " 6 " Bluish-green, calcareous laminated tuff 4 " 0 " Limestone, slightly ashy 1 " 8 " Green tuff 0 " 2 " Fine-grained decomposed tuff 0 " 4 " Green tuff, obliquely laminated 1 " 7 " Fine laminated tuff 0 " 8 " Green compact tuff 1 " 8 " Obliquely laminated shaly tuff 0 " 10 " Concretionary ashy limestone 1 " 4 " Compact ashy limestone 2 " 0 " Green shaly tuff, much weathered 0 " 5 " Ashy limestone 0 " 7 " Compact green tuff more than 4 " 0 " -------------- 41 feet 9 in.