J. J. Harris Teall—Cheviot Andesites and Porphyrites. 149 
abundant, and they undoubtedly occur as lavas in association with 
tuffs of a similar andesitic character. The vesicular and scoriaceous 
portions of the original flows are now amygdaloidal, owing to the 
deposition of chalcedony, jasper, and quartz, together with certain 
green minerals in the original cavities. A cursory examination of 
the thin sections of the porphyrites shows at once that they are closely 
related to the andesites; that some of them in fact are andesites, 
whilst others are merely altered andesites. The altered forms very | 
largely predominate, and the question arises, why is it that certain 
andesites have been preserved from alteration, whereas others have 
been so profoundly modified as to have lost almost all traces of their 
original character ? No doubt the original rocks varied in character 
as do the Tertiary andesites of Hungary ; some were vitreous, others 
semi-vitreous, and others compact and crystalline. It is these latter 
types which appear to have suffered most during the lapse of time ; 
the former, in which the constituents were imbedded in a highly 
acid glassy base, having retained their original characters to a sur- 
prising extent. As the rocks now to be described vary in character 
much more than those referred to in the previous paper, I propose to 
select a few of the more characteristic specimens and describe them 
individually. 
Thompson’s Walls, near Yetholn.—My attention was called to this 
rock by Prof. Lebour, who kindly gave me a specimen. I have 
since visited the locality. ‘The rock is exposed in an old quarry on 
the hill-slope behind the houses at Thompson’s Walls, and a similar 
but more altered rock occurs at Goldsmouth Hill, about two miles 
S.S.W. of Thomson’s Walls. Macroscopically it consists of a dark 
purple or black compact matrix, throughout which porphyritic felspars 
of triclinic species are abundantly scattered. The pyroxenic con- 
stituent cannot be easily recognized with a hand lens. The specific 
gravity of the rock is 2°67. The microscopic appearance of a thin 
section is represented in the Plate, Figure 1. The rock from which 
the section was prepared is very fresh, and thoroughly entitled to the 
term andesite. I will now give a description of that portion of the 
section which is represented in the Figure. Nearly in the centre of 
the field of view is a somewhat rounded grain of felspar, evidently 
a fragment of one of the larger porphyritic felspars. Many other 
similar grains may be recognized in the section. To the south-east 
of this grain, supposing the figure to represent a map, is a section of 
a felspar crystal with definite crystalline boundaries; inclusions of 
the ground-mass are abundant and their characters are well repre- 
sented in the Plate. Under crossed Nicols the section is seen to be 
striated, so to speak, from H.N.E. to W.S.W.; but the striations are 
somewhat ill-defined, owing to the fact that the section is cut very 
oblique to the twinning plane. 
On the west side, and with its longer edge parallel with a diameter 
of the figure, is an elongated rectangular section taken out of the 
zone of the macrodiagonal axis. In this case the twin striation is 
sharply defined. If we use the letters a and b to indicate the two 
sets of lamella, then the character of the twinning may be described 
