BY D. MAWSON. 585 



residual central space (after primary crystallisation) and probably 

 at the same time most extensive laterally, allowing access of 

 material from distant sources and obtaining continued service- 

 ability as trunk channels for circulation. 



The metamorphosing solutions may have been characterised by 

 two stages; firstly, those producing uralitisation of the pyroxene 

 and affecting the sanidine by corrosion and deposition of foreign 

 dust-like matter along the cleavage traces; secondly, powerfully 

 carbonating solutions reducing especially the alkali-iron minerals 

 to simple compounds. It is entirely more probable, however, 

 that as both effects are commonly seen represented in the same 

 slide and often apparently in process together, that they have 

 been wrought not by different vapours and solutions, but only by 

 different phases of action, either singly or in conjunction, of one 

 active menstruum 



The ultimate results of metachemisation have been to produce 

 a secondary arf vedsonite-riebeckite amphibole from the primary 

 segirine (uralitisation); to convert the amphibole both primary 

 and secondary to heterogeneous paramorphs chiefly composed of 

 siderite, calcite, silica and a small quantity of fluorspar, and in 

 addition, where carbon dioxide has not been present in sufficient 

 quantity, some brown oxide of iron; powerful corrosion of the 

 sanidines, often only dusty paramorphs remaining; choking of 

 former passages and miarolitic and drusy cavities by deposition 

 of chalcedony and quartz, usually stained by oxide of iron and 

 enclosing other less abundant secondary minerals, as fluorspar, 

 calcite, &c. 



It is necessary to postulate final escape for the active circu- 

 lating waters in order to explain the complete disappearance of 

 alkalies set free from the silicates affected. 



The carbon dioxide forming alkaline carbonates so potent in 

 attack on the silicates has, without a doubt, originated from the 

 effects of intrusion into the thick coal measures below. 



(c) Thorn exhibiting well marked flow structure; at the same 



time a strongly developed prismatic border zone is persistently 



present. 



45 



