350 Scientific Proceedings, Royal Dublin Society. 



pieces. Quartz is present as a secondary mineral, sometimes in 

 well-formed limpid crystals. The rock is much altered and 

 decomposed. 



Remarks. — This closes the list of rocks submitted to me by 

 Mr. Aitken. The result of comparing the properties of the 

 dolerite No. X. with the last five dolerites is of considerable 

 importance, and strongly emphasises the legitimacy of referring 

 to petrological examination in such cases. The rapid destruction 

 of the last five dolerites is at once ascertained to be due to the 

 decomposed state of the rocks, especially of the augite. On this 

 mineral the coherence of an ophitic rock depends. If it is turned 

 to soft minerals or veined by such, the coherence of the stone is 

 lost; and it must inevitably soon break up to mud. Number X. is 

 a rock of characters practically identical with the defective rocks, 

 but possesses a perfectly fresh and unaltered augite. Thus the 

 whole mass of felspars and augite is held together as one coherent 

 substance. Hence its resistant properties. 



It is apparent, from Mr. Aitken's observations, that an ophitic 

 dolerite having a fresh augite is an excellent metal in every way, 

 whereas one with softened augite should be rejected. 



Quartzite. — Geological formation, Cambrian. From Howth, 

 County Dublin. This is a yellowish rock, almost pure quartz, 

 and very hard and tough under hammer, breaking with a rough 

 fracture. It has no appreciable cleavage. 



It is very durable, and makes excellent roads of white colour. 



Microscope. — Is practically quartz throughout, consisting of 

 quartz granules cemented together by a cement of quartz. 



