102. J. J. Harris Teall—Cheviot Andesites and Porphyrites. 
activity during the deposition of the Lower Old Red Sandstone 
strata, when the porphyrites and tuffs of the Pentland, Ochil, and 
Sidlaw Hills! were formed, and it becomes therefore almost certain 
that the Cheviot volcanic rocks belong to this period of geological 
time. 
It may be mentioned in passing that there occur about Kelso 
igneous rocks of Tuedian age, that is, of later date than those now 
under consideration. No doubt these belong to the period of 
voleanic activity, of which Prof. A. Geikie speaks as follows :*— 
“‘The most persistent zone of volcanic rocks in the whole of the 
Scottish Carboniferous series is that which succeeds the lower or 
red sandstone group of the Calciferous sandstones, composed of suc- 
cessive sheets of porphyrites and tuffs, it sweeps in long isolated 
ranges of hills from Arran and Bute on the west, to the mouth of 
the estuary of the Forth on the east, and from the Campsie Fells on 
the north to the heights of Ayrshire, and still further south in 
Berwickshire, Liddesdale, and the English border. These volcanic 
sheets sometimes reach a thickness of 1500 feet.” I have one 
specimen of Kelso porphyrite, which was given to me by Mr. Clough. 
This rock will be described later on. It differs materially from any 
Cheviot rock known to me. 
Microscopic ANALYSIS or CoryioT ANDESITE (PITCHSTONE 
PoRPHYRITE). 
(1). The following remarks are based on the examination of 
sections prepared from specimens collected by myself in the Coquet, 
about a quarter of a mile above Windy Haugh, and near Carl Croft, 
two miles and a quarter further up the valley; in the Usway about 
Fairhaugh ; in the Allerhope Burn, at a point about one mile from its 
junction with the Alwin; and from the Alwin boulders above referred 
to. Mr. House, of Newcastle, kindly supplied me with a specimen of 
the Yetholm rock. It will thus be seen that specimens have been 
collected from a tolerably extensive area. The first point to notice 
is the uniformity in the character of the rock. This uniformity is so 
great that it will be unnecessary, except in special cases, to refer to 
particular localities in giving a description of the rock. An examina- 
tion of the sections at once proves that we are here dealing with 
a group of comparatively unaltered andesites. Messrs. Voight and 
Hochgesang, of Gottingen, have supplied me with a series of fifteen 
sections of the augite andesites of Tokaj (Hungary) and Santorin, 
and the resemblance between the Cheviot sections and this series is 
most striking. Indeed, one section of an Alwin boulder can scarcely 
be distinguished from a section of the 1866 lava from Aphroessa. At 
least, if the labels were removed, and the sections ground to the same 
uniform thickness, and remounted on similar slides, I doubt if I 
could refer them to their proper localities. Similar minerals occur 
in nearly the same relative proportions. They are similarly developed 
1 Prof. A. Geikie states that these voleanic rocks (porphyrites and tuffs) attain 
a maximum thickness of 6000 feet. Text-Book of Geology, 1882, page 707. 
* Text-Book of Geology, page 739. 
