674 ME. F. B. C0WPEE EEED ON THE [N~OV. I9OO, 



mechanical disturbance with the surrounding rocks, and are them- 

 selves in several cases penetrated by later intrusions. 



d. Intrusions of Dolerite. 



Not many intrusions of true dolerite occur along the coast, 

 most of the dark-greenish rocks which resemble them at first sight 

 proving to be decomposed keratophyres. The true dolerites occur 

 mostly as small irregular veins or tongues, and rarely as dykes. 



At Newtown Head (fig. 9, p. 671) it has been remarked that 

 irregular veins of fine-grained dolerite penetrate the coarse diabase 

 of the headland. Some of the veins might be termed andesitic 

 dolerites, but all are earlier than the felsite which breaks across 

 them. Most of the other irregular intrusions that traverse the 

 bedded Ordovician rocks in this locality are much less basic in 

 character, and must be classed with the trachytes or andesitic 

 trachytes. 



From here we may pass to Knockmahon, where a com- 

 plicated group of intrusive veins and irregular sheets of dolerite 

 and diabase is exhibited in the cliffs and series of small coves between 

 the villages of Knockmahon and Bunmahon. Several more or less 

 distinct varieties of these rocks, sometimes with columnar jointing, 

 are here present, and their mutual relations are intricate ; but they 

 apparently belong to one general period of eruption which is 

 certainly later than that of the augite-porphyrite and purple slates 

 (see p. 677), both of which they pierce (fig. 8, p. 668). I do not 

 think that this group of intrusions belongs to the same category or 

 age as the great diabase-sheets to the east. Rather would I look 

 upon it as a recrudescence of the local igneous activity which first 

 led to the extrusion of the augite-porphyrite ; and we know that it 

 occurred earlier than the formation of the felsitic vents and dykes 

 at this spot, since the latter pierce all the other rocks. 



e. The Smaller Intrusive Veins. 



There is a widespread development of irregular intrusive veins, 

 mostly of small size and in a decomposed condition, which have 

 been injected into the felsites, etc. prior to the formation of the 

 basic sills just described. In many cases the original characters 

 of these rocks are extremely indistinct, and their true nature can 

 with difficulty be made out, or remains uncertain, owing to their 

 advanced state of decomposition and the abundance of secondary 

 minerals. The majority are pale-grey or greenish -grey in colour, 

 with a rough irregular fracture, and are easily distinguishable from 

 the felsites by these features, as well as by their behaviour in the 

 field. 



Several definite petrological types are distinguishable, and though 

 at first they were all provisionally assigned to the andesites, it has 

 been found that they are mostly more acid in character. Some 

 of the veins, by their dark-green coloration and macroscopic 

 appearance, resemble even dolerite, but many of these are probably 

 keratophyres, and their microscopic characters support this view. 

 The majority appear to be rather trachytes or andesitic 



