66 Scientific Proceedings, Royal Dublin Society. 



is certain that in heavy traffic such sets would rapidly crumble 

 away. 



The intersertal structure of certain basalts and diabases is one 

 in which a ground-mass of small felspars encloses the larger con- 

 stituents. If glass enters as a considerable constituent of the 

 ground-mass, the durability of the rock is very doubtful. A little 

 residual glass in grains need not occasion the rejection of the 

 stone. But, as a bonding material, it should not be trusted. 



A type of structure not uncommon in basalts and dolerites, and 

 very usual in diabases which are poor in felspar, is the ophitic. 

 Here felspars are included in large plates of augite (or other 

 coloured mineral) ; the felspar being idiomorphic (possessed of 

 its own proper crystalline faces), while the containing mineral is 

 usually allotriomorphic (without regular faces). Doubtless the 

 combination of felspar and augite constitutes a fairly hard sub- 

 stance ; but whether due to the cleavage of the augite, to its some- 

 what inferior hardness, or to its liability to undergo alteration, 

 it is not resistant under the wear of the streets, and must be 

 regarded as playing the part of a substance of intermediate hard- 

 ness only. 



In some granites, granite porphyries, quartz-diabases, etc., the 

 intergrowth of quartz and felspar (pegmatite) affords, in the grano- 

 pliyric structure, a very durable and tenacious matrix to the 

 grains of prior consolidation. If present in large masses, it is very 

 certain to lead to the development under wear of polished slippery 

 prominences. 



The granites often show a structure designated miarolitic : 

 that is, possess numerous cavities, in part in-filled with idiomorphic 

 individual crystals. Such faults must, of course, be avoided. 

 Basalts are often vesicular ; and although the vesicles may often be 

 filled up with various secondary minerals, these are, in general, 

 soft and fragile (zeolites, calcites, chlorites) ; and even when hard 

 (chalcedony), the existence of these secondary materials denotes 

 generally far-gone changes in the rock of injurious nature. This 

 amygdaloidal structure is to be condemned. 



Any cleavage or joint structure is sure to result in the 

 disintegration or fracture of the set under wear. Cleavage 

 structure may be superinduced in fine-grained rocks, whether 

 sedimentary or igneous, under the influence of certain dynamical 



