64 Scientific Proceedings, Royal Dublin Society. 



Coherence principally involves the bonding of the mineral 

 grains or crystals ' — in fact the structure of the rock. 



The igneous rocks — crystallizing, as they do, progressively 

 from a magma — in general, possess the requisite bonding. Some 

 igneous-rock structures are better than others. 



The granitoid structure, the typical structure of the deep-seated 

 rocks — e.g., of the granites, syenites, diorites, and gabbros — is 

 perhaps the best generally available. Here the rock is completely 

 crystallized into minerals which closely inter-fit on more or less 

 irregular surfaces. That is, the true crystalline faces are but 

 partially developed, the development of the one mineral substance 

 having interfered with that of the other. The grains are typically 

 of about equal dimensions. 



In the granites, the quartz, in nearly all cases, plays the part 

 of ground-mass, is the last solidified, and is hence moulded around 

 the other grains. The compact inter-fit of the felspars, one with 

 another, and with the quartz plays an important part both to the 

 advantage and sometimes disadvantage of the granites as paving- 

 set material. On the score of durability, at any rate, a compact, 

 fresh granite is very certain to prove satisfactory. 



The rock, however, requires careful examination for a too far- 

 gone alteration of the felspars. Thus, if the felspar has been 

 generally converted into kaolin, or replaced by chlorite, etc., it 

 must be remembered that the hardness and cohesion of the rock 

 now depends on the quartz alone. Such a rock is likely to break 

 up rapidly. It may be remarked, in anticipation, here that a 

 mere cloudiness of the felspars is not inconsistent with consider- 

 able durability. A small quantity of the alteration-products will 

 give rise to considerable clouding, as observed in4he microscope, 

 and yet leave the crystal sufficiently resistant. The dangerously 

 broken-down felspar becomes quite inactive towards polarized light, 

 and, in the hand-specimen, is easily cut with the pocket-knife, and 



1 The toughness of the mineral substance itself, its molecular honding, of course 

 also enters the question. It is difficult to define this. Thus, the amphiboles display a 

 resistance to attrition which, under certain circumstances, is greater than the felspars, 

 although the former are the softer minerals. To see this, observe the surface of a 

 well-polished diabase or diorite, when the hornblende will be seen to stand out above 

 the felspar. It does not follow, however, that under the conditions, affecting the 

 set, the amphiboles will show the greatest resistance. 



