J. H. Teall — Cheviot Quartz- Fehites and Augite- Granites. 117 



iferous rocks of North Northumberland as well as the igneous rocks 

 of the Cheviot district, and are intermediate in composition between 

 basalts and andesites, may be referred with a considerable amount of 

 confidence to the Tertiary period. 



In the present communication it has been shown that the district 

 about Hedgehope, Comb Fell, Staindrop and Linhope Spout is 

 occupied by holocrystalline, non-porphyritic rocks of the plutonic 

 type. Are these rocks due to the consolidation, beneath the surface, 

 of the magma which produced the andesitic lavas and tuffs ? Do 

 they occupy the same position in relation to the Cheviot lavas as the 

 gabbros and granites of Mull do to the lavas of Mull according to 

 Prof. Judd ? The evidence available is not so complete as we could 

 wish, but, such as it is, it points decidedly to an affirmative answer to 

 the above questions. 



The augite-granites are evidently not characterized by a high 

 percentage of silica. Some of them strongly resemble syenites in 

 appearance ; but as quartz is present in all my specimens, the term 

 syenite cannot be applied to any of them. True augite-syenites 

 may of course occur in the district, as my observations were necessarily 

 of a very limited character. The analyses of the Vosges augite- 

 granites show that these rocks belong to the intermediate, rather 

 than to the acid class ; and that the bases, if we except the alkalies, 

 are present in approximately the same relative proportions as in the 

 Cheviot lavas. With regard to the alkalies, I would remark that we 

 have no analyses of the Cheviot plutonic rocks ; but it is certain that 

 the orthoclase is rich in soda, and that plagioclase of the oligoclase- 

 andesine type is sometimes present in greater quantity than the 

 orthoclase. I have little doubt that in some of the plutonic rocks 

 the relative proportions of the two alkalies is the same as in the 

 normal andesites. Again, Mr. Waller's analysis of a porphyrite from 

 Shillmoor^ shows an excess of potash over soda. 



There is another point in connexion with the volcanic history of 

 the Cheviot district in Old Red Sandstone times which seems worthy 

 of special attention. Why did the quartz- felsites sncceed the 

 andesites ? 



One of the most fascinating problems of geology is to account for 

 the variation in the composition of the material erupted at different 

 periods in the volcanic history of a district. Now in considering the 

 possible causes of this variation careful attention should be paid to 

 the changes brought about in the composition of a magma by the 

 successive crystallization of different constituents. 



Information on this subject may be obtained in at least three 

 different ways : (1) by observing the order in which minerals have 

 crystallized in different magmas ; (2) by examining the chemical and 

 mineralogical composition of the ground-masses of porphyritic rocks ; 

 (3) by examining the so-called contemporaneous veins which are 

 especially characteristic of plutonic rocks. 



Prof. Rosenbusch^ and others have shown that as a general rule 



1 Geol. Mag. Dec. II. Vol. X. p. 151. 



2 Neues Jahr. 1882, ii. p. 1. 



