NOTES EXPLANATORY OF THE PLATES. . 357 



position. The nature and appearance of these slaty rocks, affords additional evidence of the same 

 fact. They have evidently been formed from the decomposition and detritus of the igneous rocks. 

 There is a quarry about a mile to the NE. of West Linton, where the nature of both kinds of rocks 

 can be very closely examined, and their junction distinctly seen. The igneous rocks consist of a brown- 

 ish-yellow claystone or felspar, which easily disintegrates. Upon it, lies a series of slaty strata, con- 

 sisting of claystones and sandstones, which slope away from the hill top (where the quarry is) at an angle 

 of about 8°, and they are in no degree indurated by the igneous rock on which tliey rest. It is impos- 

 sible then to doubt, that the greywacke and felspar in the Pentland Hills, — or at least in a considerable 

 part of them, — existed previous to the deposition of the carboniferous strata. 



The only way of solving the difficulty, is by supposing, that there were two eruptions of trap in 

 that part of the district, — one previous to the deposition of the old red sandstone, and the other subse- 

 quent to the deposition jf the carboniferous strata. The first eruption would be of the felspathic rocks; 

 which brought up with it the greywacke in this part of the district, as it did also along the Lammer- 

 muir range. During this first eruption, it is probable that there was neither greenstone nor basalt evolved, 

 as we do not find any pebbles of these rocks in the old red sandstone. We know, indeed, aliunde, that 

 the great proportion of these rocks in the district were not, in fact, erupted till after the deposition of 

 the cm-boniferous strata ; — so that it is extremely likely, that the whole of that class of igneous rocks 

 existing among the Pentlands, appeared only at this later epoch. It is to a certain extent a confirmation 

 of this opinion, that the part of the Pentland Hills where the stratified rocks are most highly inclined, is 

 towards the east, where Greenstone most abounds among them, and where they are in close vicinity to 

 Arthur's Seat, Craiglockhart, &c. If this inference be correct, then the two eruptions might be designat- 

 ed the Felspathic and the Augitic, being, as they are, characterized and distinguished by a difference 

 in the nature of the volcanic matter indicated by these names. In many parts of the Pentland Hills, 

 Greenstone and Basalt occur, in the form both of hills and of dykes. These dykes are seen traversing 

 the felspar and greywacke rocks, — as well as the old red sandstone and carboniferous formations ; — 

 so that there can be no doubt, I think, that they must have given the Pentland Hills, especially in their 

 eastern parts, an additional upheave. 



Whilst such is my own opinion, I am aware that it is not concurred in by several geologists who 

 have examined the Pentland Hills with great care and assiduity. Mr Maclaben is about to publish a 

 geological account of these hills, which will probably contain ample materials for a satisfactory decision 

 of the above question. 



The basalt and greenstone rocks are not nearly so abundant in the district as in other coal-fields. 

 The direction towards which the trap flowed seems to have been from Arthur Seat or its neighbourhood ; — 

 for the dykes on the east side of it, all thin away in that direction, — whilst those on the opposite side 

 thin away to the west. The Niddry dyke terminates near Brunstain House. The Cock«nzie dyke 

 terminates in the field NE. of Lanridge. Between Red Coll and Lanridge, its thickness is only 50 

 feet. 



The Sections on plates XV. and XVI. hardly require observation, — as the explanations already given, 

 in regard to the colours on the Map, are applicable equally to them. These sections are not entirely 

 imaginary. The position and direction of the slips, — the dip of the strata and their relative distances fi'om 

 each other, with the points where they crop out to the surface, have all been laid down, according to the 

 fact. There may be some error in the relative distances of the strata from each other, — and also in the 

 identification of the lowest bed of limestone. But as the object was to give a general idea of the form 

 of the two basins, as they now exist, and of the manner in which the strata have been dislocated, these 

 possible errors seem immaterial. 



The Sections on Plate XVII. are intended to exhibit a vertical section of the whole strata, — in the 

 district from top to bottom. They have been necessarily divided on the Plate, as it would have been 



