_Lavas._--The lavas occur in numerous sheets, sometimes separated by thin partings or thicker beds of tuff and volcanic conglomerate. On the northern rim of the basin Mr. G. H. Kinahan has described the volcanic series east of Shehan's Cross-roads as composed of six zones of tuff, each bed varying from about 50 to 250 feet in thickness, alternating with as many sheets of lava ranging from 27 to 180 feet in thickness, the total depth of tuff being estimated at nearly 500 feet and that of the lavas at about 800 feet.[79] Some of these tuffs are coarse conglomerates or agglomerates, with blocks of lava occasionally 10 feet long. [Footnote 79: Explanation of Sheet 144, p. 28.] Some of the lavas in the lower volcanic group are andesites quite like those of the plateau series in the Carboniferous system of Scotland. Externally they appear as dull reddish-brown or purplish-red compact rocks, with abundant porphyritic felspars scattered through the fine-grained base. They are generally much decomposed, showing on a fresh fracture pseudomorphs of chlorite, hæmatite and calcite after some of the minerals, with abundant hæmatitic staining through the body of the rock. Amygdaloidal structure is commonly developed. These andesites, when examined microscopically, were found by Mr. Watts to present the characteristic base of minute felspar-laths with magnetite and enstatite, and with porphyritic crystals, often large, of zoned plagioclase, as well as of ilmenite and hæmatite. [Illustration: Fig. 194.--Section of the volcanic escarpment, east of Shehan's Cross-roads, south of Limerick. 1. Limestone; 2 2. Tuffs; 3 3. Lavas.] But besides the andesites there occur also, and, so far as I have observed, in larger number, sheets of true basalt. This rock is typically black, exceedingly close-grained in the central portion of each sheet, but becoming highly slaggy and vesicular along the upper and lower parts. Under the microscope it is found to contain granular augite and magnetite, set in a more or less devitrified glass, with microlites of felspar, porphyritic plagioclase, serpentinized olivine, and some well-marked augite. These rocks form distinct escarpments along the northern rim of the basin as in the foregoing section east from Shehan's Cross-roads (Fig. 194). From the summit of this ridge, which is about 600 feet above the sea, the eye looks northward over the plain, across which low outliers of the volcanic series are scattered, and southwards across the basin to the corresponding line of volcanic heights forming the southern rim.