CHAP. XXI 
CHEVIOT HILLS AND BERWICKSHIRE 
337 . 
This volcanic pile, consisting mainly of bedded andesites which rest 
unconformahly on the upturned edges of Wenlock shales and grits, presents 
a most typical display of the lavas of the Lower Old Ked Sandstone. These 
rocks range from vitreous or resinous pitchstone-like varieties to coarsely 
porphyritie forms, on the one hand, and to highly vesicular and amygdaloidal 
kinds, on the other. Analyses of some of these rocks, and an account 
of their petrography, have already been given. 
The lavas are often separated by thin partings of tuff, and their upper 
surfaces show the fissured character with sandstone infillings, so character- 
istic among the lavas of “ Lake Caledonia.” ' Tuffs form a very subordinate 
part of the whole volcanic series. One of the most important bands is 
a thick mass at the base, of the series, lying immediately on the highly 
inclined Silurian shales. The fragments are generally of a fine-grained 
purple mica-andesite, often two or three feet and sometimes at least five 
feet long. Lor a few feet near the bottom of this mass of tuff, pieces of 
Silurian shale an inch in length may be noticed. Mr. Clough remarks that 
distinct bedding is not usual among the tuffs. Though no doubt most 
of the fragmental materials really lie intercalated between successive lava- 
streams, yet some of the isolated patches of coarse volcanic breccia may 
mark the sites of eruptive vents. One such probable neck has been mapped 
on the Scottish side between Coeklawfoot, at the head of the Bowmont 
Water, and King’s Seat, while others may perhaps occur among the detached 
patches that have been observed on the Northumbrian side. No thick con- 
glomerates or sandstones have been noticed in the Cheviot District. The 
volcanic eruptions appear to have usually succeeded each other without the 
spread of any notable amount of ordinary detritus over the floor of the 
water-basin. It is ditticult to estimate the total thickness of volcanic 
material here piled up, but it probably amounts to several thousand feet. 
The top of the series is not visible, having been partly removed Ijy denuda- 
tion and partly buried under the Carboniferous formations. 
It will thus be seen that the Cheviot area stands apart from the other 
volcanic districts of the Low’er Old Ked Sandstone in the great relative 
thickness of its accumulated lavas, the comparative thinness of its tuff's, and 
the absence of the thick intercalations of coarse conglomerate so abundantly 
developed among the volcanic series all over Central Scotland. But there 
is yet another characteristic in which this area is pre-eminently conspicu- 
ous. In the heart of the andesites lies a core of augite-granitite, around 
which these rocks are traversed with dykes. 
This interesting granitic boss rises into the highest summit of the whole 
Cheviot range, and covers an area of rather more than 20 square miles. 
While its petrographical characters have been described by Mr. Teall, 
its boundary has been mapped by Mr. Clougii, who found the line difficult 
to trace, owing partly to the prevalent covering of peat, and partly to the 
jagged and irregular junction caused by the protrusion of dykes from and 
into the boss. He obtained evidence that the granite has broken through 
' Clough, Geology of the Cheviot Hills, p. 16 . 
VOL. I 
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