180 STRATIGRAPHICAL GEOLOGY 



and rest directly on the Archaean rocks, their base being a breccia 

 consisting of angular lumps of quartz and schist. Above this are 

 red mudstones and a band of calcareous grit with Upper Llandovery 

 fossils succeeded by a massive conglomerate consisting mainly of 

 rounded pebbles and boulders of quartzites like those of Connemara. 

 Then comes a thick series of coarse grits alternating with green 

 sandy slates which have yielded a few fossils of Wenlock species, 

 Monograptus riccartonensis and M. vomerinus. These beds may be 

 about 2000 feet thick. Above them are the Salrock Beds, dull red 

 and green shales, with thin bands of grit, and having a thickness 

 of about 3000 feet. They contain a small species of Lingulo, 

 (L. Symondsi) with Pterincea retrqflexa and Modiolopsis complanata. 

 They probably represent the greater part of the Ludlow and 

 Downton Beds. 20 



In the Dingle promontory the ascending succession is as 

 follows 21 (see Fig. 60) : 



(1) Smerwick Beds, a thick series of red, green, and yellow 



sandstones with bands of conglomerate ; fossils rare, but 

 Atrypa hemispherica and others have been found. 



(2) Ferriters Cove Beds, greenish sands and shales with bands of 



red sandstone ; these beds are 2500 feet thick and contain 

 fossils of Wenlock species. 



(3) Croaghmarhin Beds, hard brown calcareous grits with Pen- 



tamerus Knighti and other Ludlow fossils. These dip 

 southward at a high angle and pass below a great series 

 of green and purple grits and shales which are known as 

 the Glengarriff grits, and believed to be of Devonian age. 



Here, also, there are evidences of contemporaneous volcanoes. 

 The best sections are in the cliffs near Clogher Head, where the 

 iiiterstratification of the lavas and tuffs with the fossiliferous 

 Wenlock Beds is beautifully exposed. Sir A. Geikie describes the 

 lavas as nodular felsites, the nodules being from the size of a pea 

 to that of a hen's egg, and the associated pyroclastic rocks vary 

 from fine pumiceous tuffs to coarse agglomerates. One of these 

 beds is a coarse agglomerate about 15 feet thick, consisting of large 

 blocks of felsite embedded in calcareous sandstone which is full of 

 the casts of brachiopods, crinoids, and corals. The district has been 

 recently examined by Messrs. S. H. Reynolds and C. T. Gardiner, 

 who found that some of the lavas are rhyolites, and that all are of 

 acidic character. 22 



From these facts we learn that a volcano existed in this part of 

 the Silurian Sea, and that from time to time it burst into eruption, 

 pouring forth lava streams and ejecting showers of stones and ashes, 



