116 J. S. Teall — Cheviot Quartz-Fehites and A gite- Granites. 



described by many observers, but especially by Br gf^er.^ The 

 ortboclase in these rocks, like that of the Cheviot granites, is rich in 

 soda. The augite-granites of tbe Vosges have been already referred to, 

 and as they are very closely related to the Cheviot rocks, it is necessary 

 to quote their analyses. 





I. 



II. 



SiOz 



... 61-93 ... 



... 62-09 



TiO, 



— 



•56 



AI263 



... 13-18 ... 



... 16-43 



FezOa 



3-63 ... 



2-34 



FeO 



2-31 



2-03 



CaO 



3-48 ... 



2-32 



MgO 



4-59 ... 



308 



NajO 



2-67 ... 



407 



K2O 



611 ... 



4-66 



H^O 



1-14 ... 



•85 



99-04 98-43 



I.^ Augite- Granite from Laveline. Sp. Gr. 2-723. 

 11.^ Augite-granite from Oberbruck. Analyses by Werveke. 



Eosenbusch* has also recognized fresh pyroxene in the Vosgps, 

 porphyries of Etival, Eothau, Eochesson, and in a rock wbich is 

 sometimes a quartz-porphyry and sometimes a granite from the 

 Titisee in tbe Schwartzwald. He remarks that tbe mineral is absent 

 as a rule from massive granites, but that be has observed it 

 abundantly in one case, viz. that of a rock from the Julier Pass. 



Mutual Eelations of the Igneous Eooks of the Cheviot 



District. 



The facts, so far as they are known at present, appear to show 

 tbat the first period of volcanic activity in this district was marked 

 by the eruption of immense quantities of audesitic lava and tuff. 



The andesites fall into three groups, characterized respectively by 

 the minerals hypersthene (bronzite), augite, and mica. Most of 

 them are now much altered, and to these altered forms I have 

 restricted the term porphyrite. The presence of tuflfs, vesicular 

 and amygdaloidal varieties of lava, and actual evidences of flow, 

 proves that the conditions of eruption were substantially identical 

 with those of modern times. 



The eruption of andesitic material was followed by the intrusion 

 of quartz-felsite dykes. If the magma which produced these dykes 

 ever reached the surface, it must unquestionably have formed 

 rhyolites and rhyolitic tuffs. Do such rocks occur anywhere 

 in the Cheviot District? The dykes of quartz-felsite occur in the 

 porphyrite region, so that we have conclusive evidence to show that 

 the eruption of acid followed that of andesitic material in the 

 history of the Cheviot Old Eed Sandstone volcanoes. 



The intrusion of quartz-felsite dykes appears to have been the 

 concluding phase of the volcanic activity of Old Red Sandstone 

 times ; for such dykes as those of Acklinton, which cut the Carbon- 



^ Die silurischen Etagen 2 und 3. 



^ Studien an gesteinsbiklenden Pyroxenen, Merian. 



3 Neues Jahr. 1883. I. 201. * Z.D.G.G. 1876, p. 369. 



