293 ANDESITE LAVAS OF THE CHEVIOTS. 



top of felstone lavas, alternating with volcanic agglomerates ; below 

 these is a bed of felsite resting on another felsite, which graduates 

 downward into a fine-grained granite. Under this is the coarse 

 porphyritic granite, which burst through the Cambrian rocks. On the 

 one hand, it graduates into syenitic granite, and on the other into 

 granite, consisting of orthoclase, oligoclase, quartz, and hornblende. 



Thus the granite graduates externally into sucn rocks as granite is 

 known to become when it cools rapidly, and it is overlain bv such lava 

 and ashes as would have been poured out from a volcano having a 

 granitic base. Therefore, when it is found that great lava-sheets in 

 the Scottish Lowlands are associated with the Lower, Middle, and 

 Upper Old Red Sandstone in the district of Lome, that lava streams 

 also occur upon the northern flank of the Grampians, and that the hill 

 ranges of southern Scotland are composed of volcanic rocks, it seems 

 highly probable that the Grampian range was elevated as a chain of 

 volcanic islands, 1 not unlike some of those which now occur in the 

 Indian Ocean or the Pacific. 



The activity of these volcanic centres during the Old Red Sand- 

 stone and succeeding Carboniferous periods, must have been largely 

 determined by the action of the compressing forces which elevated 

 those areas out of the ocean ; and it is worth remarking that the 

 Old Red Sandstone is a shallow-water deposit, even if it was not of 

 lacustrine origin. 



Cheviot Andesita Lavas. The Cheviot district consists chiefly 

 of quartzless porphyritic rock, such as is usually termed porphyrite. 

 Under the microscope it is a compact felsitic ground mass, usually 

 purple or red, with crystals of triclinic felspar. Volcanic ash and 

 breccia are found on both the English and Scotch sides of the 

 eruption. And a black resinous rock, which has been called pitch- 

 stone-porphyrite, occurs near Cherrytrees in Roxburghshire, in a cliff 

 near Yetholm, in the Coquet between Windy Haugh and Blindburn, 

 and in the Usway between Battleshields Haugh and Fairhaugh. 

 These rocks belong to the same group as ordinary porphyrites, and 

 are regarded by Mr. Teall 2 as ancient altered andesites, belonging to 

 the Lower Old Red Sandstone period, during which the Pentland, 

 Ochil, and Sidlaw Hills were formed. Porphyrites of later date 

 occur about Kelso, which belong to the Tuedian beds at the base of 

 the Carboniferous formation. These rocks can scarcely be distin- 

 guished from the andesites of Santorin, and Tokaj in Hungary. The 

 felspars in the ground mass form a felted aggregate of microliths. 

 The augite in most if not in all the andesites is subordinate to a 

 rhombic pyroxene which is regarded as hypersthene. An andesite 

 lava from near the summit of Ararat, contains a twinned monoclinic 

 and a dichroic pyroxene ; and this condition is found in the andesites 

 of Southern Servia, and the districts near Schemnitz, Kremnitz, and 

 Eperies. The Cheviot andesite gives on analysis 



1 Judd, Q. J. G. S., vol. xxx. pp. 295, 289, &c. 



2 J. J. H. Teall : Geol. Mag. March, April, May, 1883. 





