B44 J. J. Harris Teall—On Hypersthene Andesite. 
rise and the intermediate troughs must sink.!_ The great monoclinal 
folds defining the several mountain-flanks must gradually develope 
into magnificent overfolds, and these again in the process of time 
into overfaults, having the appearance of normal dislocations with 
tremendous vertical downthrow. Only, however, in ranges geologi- 
cally new, or composed of rock-formations of immense thickness, 
can these features be expected to be typically developed. The reader 
may collect abundant illustrations of this structure from the magnifi- 
cent maps and sections of the Uinta and Wasatch areas in Western 
America, where both these conditions appear to obtain; and may 
observe how this theory brings into harmony many of the known 
facts of the interrelations of the folding, faulting, and remarkable 
physical geography of that wonderful region, and reconciles at the 
same time some of the apparently diverse interpretations of its 
geological development put forward by its enthusiastic describers. 
In more easily convoluted regions, or in mountain chains of much 
higher antiquity, this inevitable relation of sunken trough to moun- 
tain arch is necessarily less conspicuous, and long eludes detection 
owing to the bewildering complexities of the strata involved. But 
it is by no means impossible that the long straight (longitudinal or 
strike valleys) and so-called anticlinal valleys of the Scottish High- 
lands, such as those of the Great Glen and Loch Tay, walled in by 
steep hill-slopes and occupied by lakes of profound depth, are 
nothing more than greatly depressed intermont synclinal troughs 
owing their origin to the same causes which bring about the slow 
secular elevation and approximation of their flanking ranges. 
Il.—On HyprerstHene ANDESITE. 
By.J. J. Harris Trart, M.A., F.G.S. 
A’ the conclusion of my last paper on the Cheviot andesites and 
porphyrites (Geox. Mac. Vol. X. p. 262), I called attention to 
the close resemblance between the Cheviot hypersthene andesite (or 
porphyrite) and a rock from Steinerne Mann, Nahe, which has been 
called a proterobase.2 Both rocks are black in colour and consist 
1 To this relation of sunken trough and mountain arch, there can, indeed, be no 
exception. The grand arch constituting a mountain chain is most properly regarded 
as an elevated anticlinal limited exteriorly by two depressed synclinals. Hence the 
paragraph ix. (a) 4 (p. 198) is mechanically incorrect as it stands, and should read :— 
(1X. 4.) At the foot of either bounding synclinal of a mountain range the thrust 
is wholly horizontal; the twisting or shearing force is zero or very small, and the 
resulting folds are normal (symmetrical) and regular. But as we proceed towards 
the centre of the anticlinal of the range, this horizontal thrust is combined with 
a vertical component due to the weight of the mass above; the total thrust therefore 
becomes greater, and, moreover, its direction is more or less oblique to the transverse 
section of the rock, i.¢. the twisting or shearing force becomes greater also. (The 
maximum of these latter effects occurs somewhere near the junction of the syncline 
and the anticline.) As a natural consequence the axes of the folds no longer remain 
vertical, but slope obliquely outwards, in the manner of inclined or reflexed folds. 
2 The term proterobase was introduced by Giimbel (Die palaolithischen Eruptiy- 
gesteine des Fichtelgebirges, Miinchen, 1874) for certain rocks occurring in the 
