24 Proceedings of the Royal Irish Academy. 



with caloite or a green chloritic mineral. This ash falling in showers into the 

 sea became mixed with epiclastic material, such as limestone and chert, the 

 whole finally consolidating to the compact rock which now forms the breccia 

 or ash of the district (fig. 2). 



The intrusive rocks occur for the most part 

 as solid pipes, filling up the vents from which 

 the ash was ejected. One of these intrusions 

 forms the core of Croghan Hill itself, with the 

 ash surrounding it ; and the smaller cones, 

 standing out, particularly on the eastern and 

 northern sides, as low hills round the base of 

 the main vent, were probably parasitic cones fed 

 by channels from the chief pipe of the volcano. 

 Towards the close of activity, when the explo- 

 sions which gave rise to the ash had ceased, the 

 lava rose quietly from below, and solidified in 

 the vents. An examination of these intrusions produced no evidence to 

 show that they took place at different intervals, nor any indication of 

 the successive protrusion of progressively different types of lava. The 

 petrographic types, having much in common, and not being possessed 

 of great variety, seem to point to the conclusion that they were intruded 

 simultaneously from a common magma, the small variation in composition 

 being due to local physical causes in the magma at the time of intrusion. 

 The rock varies from a blue-black fine-grained basalt to a more doleritic type, 

 where the plagioclase is distinct, and in a few instances a more basic type, 

 passing into a limburgite. All have a more or less prevalent amygdaloidal 

 structure ; the amygdales are generally composed of calcite, but often contain 

 serpentine. 



In one locality in Croghan demesne Professor Watts 1 calls attention to a 

 noticeable feature in these volcanic rocks. This is the occurrence in them of 

 lumps of a highly crystalline material quite distinct from the enclosing rock. 

 The enclosures referred to vary in size up to a foot in diameter, and are doubtless 

 blocks caught up by the lava in its ascent. These rocks are described by 

 Professor "Watts. One specimen " contains relics of garnets, surrounded by 

 rings of kelyphite, embedded in a mosaic of felspar, with a mineral which may 

 possibly be idocrase." He describes another specimen as containing " the relics 

 of garnets preserved as kelyphite, set in a matrix of quartz-grains, much 

 strained, and containing a profusion of crystals of greenish-yellow or red 



1 Geol. Surv. Ireland, " A Guide to the Collection of Rocks and Fossils," p. 38. 



