120 J. H. Teall — Cheviot Quarts-Fehites and Avgite- Granites. 



acid granophyres of the summit of the hill. Is it possible that 

 these facts may be due to an action of the kind above referred to ? 



The point, however, to which I wish to direct special attention is 

 this. The eruption of andesitic lavas and tuffs in the Cheviot 

 district was followed by the intrusion of quartz- felsite dykes. The 

 plutonic rocks of this district are traversed by veins of quartz-felsite. 

 ]f these veins be of the nature of contemporaneous veins, as I believe 

 them to be, at any rate in part, then they owe their special chemical 

 characters to progressive crystallization in a magma of andesitic 

 composition, and we may see in this a reason why quartz-felsites 

 succeeded andesites in the history of the Cheviot volcanoes. This 

 view receives strong confirmation from the fact that the ground-mass 

 of the andesitic lavas is substantially identical in composition with 

 the quartz-felsites ; as will be seen at once by comparing the two 

 analyses by Dr. Petersen with the analysis of the quartz-felsite by 

 Mr. Waller, which is given in the earlier part of this paper. 



If we compare the history of volcanic action in the Cheviots with 

 that of other districts, we are struck by many points of resemblance. 

 The succession, andesite, trachyte, rbyolite, is common to many 

 volcanic regions, including Hungary, the Lipari Islands, and North 

 America.^ 



The sequence of volcanic rocks is, however, usually complicated 

 by the introduction of basalt during the later phases of volcanic 

 activity, a fact which shows that the view adopted for the purpose 

 of explaining the volcanic sequence in the Cheviot district is not in 

 itself sufficient to cover all cases. Doubtless, as Captain Dutton has 

 pointed out, many factors are involved in the determination of the 

 sequence. 



The phenomena of the Lipari Islands, so admirably described by 

 Prof. Judd,^ furnish a very interesting case. The first period of 

 activity in this region appears to have been characterized by the 

 eruption of andesitic material. Then, as time went on, the products 

 diverged, as far as composition is concerned, in opposite directions, 

 and in modern times we see Stromboli erupting basic and Vulcano 

 acid material. The sequence can be explained if we regai'd the pro- 

 ducts of Vulcano as formed from the mother-liquor and those of 

 Stromboli as resulting from the refusion of the basic minerals which 

 have separated out of it. 



There is one interesting jooint, almost of the nature of a paradox, 

 to which attention has frequently been directed, but which it seems 

 advisable to refer to in connexion with the present subject. The 

 fusion point of basic rocks is lower than that of acid rocks, and yet 

 minerals separate out as a general rule in the order of their basicity, 

 the more basic being the first to form. How is this to be explained ? 



^ Propylite, the rock which has long been supposed to mark the first period of 

 volcanic action in Tertiary times, has been shown by the recent work of Dr. Hecker 

 (Geology of the Comstock Lode) to comprise, in America, altered forms of well- 

 known rocks, especially andesite, and a similar conclusion had been previously 

 arrived at by Dr. Wadsworth. 



2 Geol. Mag. 1875. 



