198 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 3, 
slaty structure. The action of the porphyries is similar, often merely 
changing the grey colour of the rocks to red or brown ; occasionally 
converting the greywackes into masses of hard flity slate or clink- 
stone, with distinct imbedded erystals of felspar ; or rarely producing 
no apparent action. As these porphyries are sometimes considered 
contemporaneous with the associated strata, the following horizontal 
section (Fig. 1) may have some interest. It shows the rocks, as ex- 
posed on the sides of the mountain-stream above noticed, at Priest- 
hope near Innerleithen, which runs along a fissure or fault, by which 
both the greywacke and the felspar bed have been shifted horizontally 
for eight or ten feet. On one side of the section the porphyry im- 
closes a fragment of greywacke, and on the other forms two short 
veins penetrating the adjoming strata. Similar sections are not un- 
common, and from these appearances and the changes produced on 
the greywacke, I believe that the porphyry im this district has in 
general been injected among the strata in a state of igneous fluidity. 
Some of these bedded veins run for considerable distances almost in 
a straight line parallel to the strata. Thus, one which passes through 
the hill immediately above the mineral well at Innerleithen, which 
may be regarded as an additional proof of its igneous origin, may be 
traced almost continuously for five or six miles in one direction. 
The position of these rocks, or the dip and direction of the strata, 
presents some points of interest. The direction of the beds, with 
local exceptions, is almost invariably parallel to the direction of the 
mountain-chain, or nearly east and west by compass (the variation 
being about 26° west of north). The dip again is almost constantly 
at a high angle on the northern side of the formation, from 60° to 90°, 
but as we proceed south becomes lower, till i Roxburghshire it is 
more often about 30° or 40°. The poimt to which the strata dip is 
not thus constant, bemg in Peeblesshire as often north as south, but 
in many cases towards the centre of the mountain ridges, or north on 
their south declivity and south on the north. In Selkirkshire and 
Roxburghshire again a southerly dip begins to prevail, and becomes 
more constant near the border of England. From this statement it 
appears that the direction of the beds corresponds with the great 
longitudinal valleys formerly mentioned. On the other hand, the 
direction of the transverse valleys is parallel to a series of trans- 
verse lines of division, crossmg the strata nearly at right angles to 
the strike of the beds. In consequence of these divisions the strata 
are cut into large quadrangular masses, corresponding on the small 
scale to the mountains on the large. | 
Such are a few of the more general features of this formation con- 
sidered as a whole. It however presents some interesting local pecu- 
hiarities from which several results of considerable importance seem 
to follow. Before mentionig these, however, we must describe the 
relations of this formation to the strata with which it is in contact. 
And first, there are no older formations on which it is seen to rest. 
It forms the oldest and lowest rock visible in this district, or of 
whose existence in it any indication appears. Of newer formations 
it is immediately followed by the old red sandstone, which m this 
part of Scotland seems to form merely the lower portion of the car- 
