J. H. Teall — Cheviot Quart'z-Fehites and Aug ite- Granites. Ill 



rocks, we see that they shade off on the one side into rhyolites, and 

 on the other into granites. As a whole, they occupy the same position 

 in the acid series as the diabases and dolerites in the basic series ; 

 they are intermediate between the volcanic and the plutonic rocks, 

 but without well-defined limits. 



The subdivisions as well as the general relations of the quartz- 

 porphyries may be expressed as follows : 

 Volcanic. Ehyolite. 





! Vitrophyre 





Intermediate. 



Quartz- | 







Porphyry j 



( Felsopliyi-e. 





I Quartz -felsite 



< Granophyre. 

 ( Micro -granite 



Fliitonic. 



Certain granites. 





I now proceed to give a description of the ground-mass of the 

 Cheviot quartz-felsites. Examined by ordinary light it never pre- 

 sents the appearance of a true glass. Ill-defined specks, flecks, 

 fibres, and scales of a brownish substance (ferrite), are everywhere 

 present and lie superposed over each other in the thinnest 

 sections. As a rule these are scattered uniformly throughout the 

 substance of the ground-mass, but in some instances they give 

 rise to small spherical aggregates exactly as in certain rhyolites 

 from Hungary and the Ponza Islands. Under crossed Nicols the 

 ground-mass is seen to be micro- or crypto-crystalline ; no portion 

 appears to remain dark under crossed Nicols. In one case a very 

 interesting structure occurs. The ground-mass presents a perfectly 

 normal appearance when examined by ordinary light, but under 

 crossed Nicols it splits up into a coarse-grained crystalline aggregate. 

 The boundaries of the different doubly-refracting grains are perfectly 

 irregular, and are not recognizable by ordinary light. Every quartz 

 crystal in the rock forms the nucleus of one of these grains, so that 

 the space surrounding it extinguishes simultaneously with the crystal 

 itself. This fact seems to show that the substance which has deter- 

 mined the definite optical characters of the ground-mass is quartz, 

 although other substances must be present in considerable quantity. 

 I incline to the view that we have here a case of true devitrification ; 

 but as there are no structures especially characteristic of glass, it 

 seems impossible to be certain on this point. 



In some slides the pseudo-spherulitic structure may be detected in 

 places, and this may be taken as indicating an approach to the grano- 

 phyric structure of Rosenbusch. 



Mr. Waller kindly analyzed a specimen from a point in the Coquet 

 about -^ mile above Shillmoor Farm, with the following result : 



SiOa 



67-9 



AI3O3 



15-7 



FeaOs 



3-0 



CaO 



1-4 



MgO 



1-5 



NaaO 



1-5 



K3O 



5-6 



Loss 



3-7 



100-3 



