91 



With regard to the genetic history of this rock, its micro- 

 scopical structure shows that it cannot in any sense of the 

 word be described as a lava poured out at the surface in 

 ancient times and cooled under atmospheric conditions. We 

 do not find the glass which results from rapid surface chill 

 imparted to flowing lava streams, and the crystallisation 

 indicates its formation below the earth's surface. 



Again, in most lavas, besides the large crystals brought up 

 from intra-telluric reservoirs, there is a generation of smaller 

 ones (generally felspars) springing into existence in the 

 moving vitreous magma at the moment of eruption. But in 

 our rock these two generations do not exist. There never 

 was a surface phase in its history, and though it happens to 

 be at the surface now, it is so only as the result of extensive 

 denudation. Moreover, the incipient traces of schiller 

 enclosures in the bisilicate tell us that we are dealing with no 

 superficial rock, for these signs of hydration only characterise 

 rocks which have formed under pressure at considerable 

 depths. 



The rock therefore agrees thoroughly with what is called 

 the intrusive type — intermediate between the plutonic rock 

 masses and volcanic lavas emitted at surface. To define the 

 conditions under which it has come to occupy its present 

 position belongs to the province of the field geologist. 



The German petrographers postulate geological age as a 

 classificatory principle for igneous rocks. This is not 

 admitted in England, and is responsible for considerable 

 confusion in the references to authors' descriptions. Thus in 

 Germany the same rocks would be called diabase and dolerite, 

 porphyrite and andesite, melaphyr and basalt, according to 

 the age of each pair. Within a limited area this plan is to 

 some extent applicable, but when we attempt to apply it 

 universally we find that it fails. Gabbros, dolerites, and 

 basalts may be found to shade into one another, presenting a 

 continuous sei'ies of one and the same basic rock substance 

 with textural modifications, ranging from the coarse granite 

 structure of the gabbros to the glass of basalt lavas. 

 Dolerite is on this theory a holocrystalline plagioclase-augite 

 rock intermediate in texture between the gabbros and basalts, 

 and the only use left for the term diabase is in designat- 

 ing those dolerites in which extensive chloritic and other 

 alterations have supervened. 



II. The Zeehan White Eock, a Vesicular Basic 

 Lava = Melaphyre. 



This rock varies in appearance according to the degree of 

 decomposition and to its condition as an ancient ash or a 

 tuff or a lava flow. The pieces prepared for examination 



