348 Scientific Proceedings, Royal Dublin Society. 



XL Basalt. Geological formation, d 2 B. 



This is a finer rock than the last. It is black in colour ; with 

 easily visible constituents. The fracture is fairly rough, with no 

 trace of cleavage. It is hard, and spalls rather conchoidally under 

 hammer. 



After ten years' wear is very little rounded, but shows a fairly 

 smooth surface. No chemical change visible. 



Microscope. — Ground-mass is typically intersertal. A dark, 

 dusty-looking glass is present along with fine lath-shaped felspars. 

 Phenocrysts of augite abundant, fairly idiomorphic. Olivine in 

 lesser quantity ; often idiomorphic, and veined with serpentine. 

 Ilmenite (?) in branching needles through the glass. Doubtful 

 felspar phenocrysts. 



Remarks. — Numbers X. and XI. are the most successful stones. 

 The first has roughness, toughness, and durability in its favour. 

 It is also a heavy rock (some advantage perhaps). The second is 

 less tough, but harder, and not so rough. It also is a heavy rock. 

 The results on these rocks are very valuable, as they are typical 

 stones of their kind, and ought to afford a safe criterion in selecting 

 among igneous rocks where these varieties are available. But 

 the rocks to be now described show that all dolerites are not equally 

 valuable. 



XII., XIII., XIV., XV., XVI., Dolerites. Tough, but deficient 

 in hardness : go quickly to mud on level stretches under traffic in wet 

 weather, fairly good material on steep hills. No. XII. makes excellent 

 kerbs, channels, and, in a lesser degree, paving-sets. (T. A.) 



XII. 1 Geological formation, Ts c i- G.B. 



A dark-grey rock with glistening fracture, finer than X. in 

 texture, but shows a fairly rough fracture. After nine years' 

 wear is not very much rounded, and surface still rough. A fresh 

 surface on the used stone shows no visible change within. 



Microscope. — This is a dolerite, without olivine, or very little. 

 The rock is made up of felspars of the usual type, imbedded 

 ophitically in a very much decomposed augite. The felspar is 

 fresh. The augite is changed for the greater part to a cloudy 

 grey substance, among which is much chlorite. The augite 



iDyke extending from Glenfaig (Perthshire), through Fife, to near Cupar: 

 40 feet to 60 feet wide. (T. A.) 



