J. J. Harris Teall—Cheviot Andesites and Porphyrites. 101 
out which are scattered numerous crystals of felspar, mostly belonging 
to triclinic species. The ground-mass varies in colour; the two 
principal shades being dark purple and red. ‘These porphyrites are 
evidently more or less altered rocks and sometimes the alteration 
has been carried so far as to make it almost, if not quite impossible 
to recognize their original nature by macroscopic examination. 
Amygdaloidal varieties are not uncommon, and in these agates are 
frequently found. In addition to the ordinary porphyrites, there 
occur masses of volcanic ash and breccia,! and also a remarkable 
rock which has been called pitchstone-porphyrite. This is dark 
almost black in colour, with a well-marked resinous lustre and a 
specific gravity lying between 2°53 and 2°62. It is porphyritic in 
texture; the large crystals consisting of a fresh glassy-looking 
triclinic felspar. Red veins usually traverse the rock in different 
directions. ‘The boulders lying in the streams are covered with a 
thin light-coloured crust due to weathering, but at a distance of 
a quarter of an inch from the surface, or even less, the fresh un- 
altered rock is always found. 
Mr. Tate mentions this rock as occurring near Cherrytrees, in 
Roxburghshire, and again in a conspicuous cliff near Yetholm. It 
has been used in the construction of the church at Yetholm. My 
attention was first directed to it by Mr. Clough’s paper, and since 
that paper was read I have visited several of the localities mentioned 
by him. The rock may be seen in the Coquet at several points 
between Windy Haugh and Blindburn, and in the Usway between 
Battleshields Haugh and Fairhaugh. Large boulders occur in the 
bed of the Alwin above Clennel; a fact which proves the existence 
of the rock within the watershed of that river. About one mile up 
Allerhope Burn, a tributary of the Alwin, a rock of the same type, 
but having a dull semi-resinous lustre, occurs. 
That the pitchstone porphyrites, or andesites as I should prefer to 
call them, belong to the same series as the ordinary porphyrites, is 
clearly shown in the sections in the Coquet and the Usway. ‘The 
evidence, therefore, already on record as to the age of the latter will 
serve to fix that of the former. In “Good Words” for 1876, p. 85, 
Prof. Geikie states that at Hindhope the basement beds of the por- 
phyrite series are a true volcanic ash or breccia, and that they rest 
unconformably on the Silurian greywackes. Mr. Clough describes 
a similar succession as existing between Philip and Makendon on 
the English side of the border. The rocks which directly overlie 
the porphyrites belong to the basement beds of the Carboniferous 
formation—the Tuedian series—and they are described both by Prof. 
Geikie ? and Prof. Lebour® as consisting in places of conglomerates 
made up in part of pebbles of rolled Cheviot porphyrites. There 
can, therefore, be no doubt that we are here dealing with a group of 
igneous rocks of Post-Silurian and Pre-Carboniferous age. 
We know that Central Scotland was the scene of intense volcanic 
1 These were recognized on the Scotch side by Prof. James Geikie, and on the 
English by Mr. Clough. 
2” Good Words, 1876, p- 265. 3 Geology of Northumberland, p. 43. 
