184 KYXASTON : VOLCANIC ROCKS OF THE CHEVIOT HILLS. 
(1.) The Andesites. The lavas, formerly termed porphyrites, are 
throughout of Andesitic composition and structure. As 
a rule they are highly vesicular and a good deal 
weathered. Fresh varieties are, however, occasionally 
met with, and we may mention the glassy andesite, seen 
in the river Coquet, about three miles above Alwinton. 
The rock has been described by Mr. Teall in the 
Geological Magazine. It is black, with the lustre of 
a pitchstone, and traversed by narrow reddish veins of 
silica. It is an enstatite-andesite, and contains numerous 
small porphyritic crystals of enstatite in addition to 
augite and plagioclase felspar, in a glassy groundmass. 
The more common types of andesite may be well studied in 
any of the crag or burn sections in that portion of the 
district surrounding the inner granite mass. They are 
augite -and -enstatite -andesites, varying in colour from 
almost black to purple, according to the degree of 
weathering which they have undergone. In texture they 
appear sometimes compact, with numerous porphyritic 
felspars, and sometimes very vesicular, the vesicles being 
filled with silica, usually in the form of agate. These 
vesicular or amygdaloidal varieties are well seen in the 
southern parts of the Cheviots in the neighbourhood of 
Alwinton, and small agates, representing the amygdules 
of disintegrated rocks, are common as rounded pebbles 
in the bed of the Coquet. The lavas are also well 
seen in the lower part of the Langlee valley, near 
Wooler, in the Carey and College burns further west, 
and in many other parts of the district. 
Volcanic rocks of fragmental origin do not play any con- 
siderable part among the erupted products of the 
Cheviots. Exposures of tuff are seen to the west of 
Ingram, which in parts is a reddish volcanic breccia 
of striking appearance. In the churchyard at Ingram 
a slab of this rock composes the headstone to the grave 
of Mrs. Allgood, the wife of the former rector. 
