| 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. |
The tuffs which in the southern part of the basin underlie the less basic lavas differ in some respects from those which further north are associated with the Upper Limestones. They are green, sometimes dull purplish-red, finely granular rocks, made up in large part of andesitic debris. They are full of loose felspar crystals, minute, somewhat rounded and subangular lapilli of andesite or some less basic lava, together with bits of grit and baked shale. Though generally much decomposed, they are sometimes compact enough to be used for building-stone. Under the microscope these tuffs are seen to abound in andesite-lapilli, with a few pieces of felsitic rocks enclosed in an opaque base, through which are scattered broken felspars and occasional vesicular lapilli.
Fig. 193.—Section in quarry on roadside east of Limerick close to viaduct of the Limerick and Erris Railway.
1. Limestone; 2. Calcareous tuff; 3. Ashy limestone or calcareous tuff.
The tuffs around Limerick, interbedded with the Black (Upper) Limestone, are distinguished by a scarcity of andesitic debris, by their persistent dull greenish-grey colour, and more particularly by the abundance of minute lapilli and larger fragments of an epidote-green, finely vesicular, easily sectile basic pumice. Under the microscope much of this material is found to be an altered basic glass of the nature of palagonite. These tuffs are in evident relation with the more basic lavas that accompany them. The manner in which they alternate with the black limestone shows that the conditions for the eruption of this more basic detritus continued to be very similar to those that existed when the andesitic tuffs were ejected. As a good illustration of this feature the accompanying section ([Fig. 193]) is given from a quarry on the side of the high-road between Limerick and Annacotty. The total depth of strata here represented is about 15 feet. The black limestone at the bottom is a tolerably pure calcareous rock. It is divided into bands by thin partings of a fine greenish calcareous tuff, each marking a brief discharge of ashes from some neighbouring vent. Half-way up the succession of strata, the ashy material rapidly increases until it usurps the place of the limestone, though its calcareous composition shows that the accumulation of calcareous sediment had not been entirely suspended during the eruption of ash.
Among these tuffs I have noticed fragments of fine, dark, flinty felsite, grit and other rocks. The stones are for the most part small, but vary up to blocks occasionally a foot in diameter.
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.
[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.