i9i4" I 9 I 5-] Blackford Hill and its Agates. 1 67 



marls, oil-shales, coals, limestones, with lavas and volcanic 

 tuffs interbedded, strata which were in turn invaded from 

 below by numerous intrusions of dolerite. It was a period 

 of slow and almost continuous subsidence of the earth's 

 crust. 



Excluding the Glacial boulder-clay, with the Carboniferous 

 rocks the positive record of geological time in our area comes 

 to an end. Whatever Mesozoic or Tertiary deposits may have 

 been laid down have disappeared. It is possible that the 

 country in the neighbourhood of Edinburgh was a land area 

 suffering almost continuous denudation throughout the immense 

 interval of time between the deposition of the red beds of 

 Dalkeith and the boulder-clay which covers them, a period 

 measured in geological chronology by myriads of extinct 

 species, and which Mr Goodchild reckoned at over three 

 hundred million years. 



" Les plis se reproduirent aux memes places," said Marcel 

 Bertrand, and here was no exception. The crustal move- 

 ments at the close of Lower Old Eed Sandstone age were 

 surpassed in magnitude by those of post-Carboniferous age. 

 The Old Eed volcanic rocks were arched up along an axis 

 roughly north-east to south-west, sharply folding the hori- 

 zontal strata of sedimentary rocks on the south-east. Master 

 faults were developed on either side ; on the north-west the 

 Colinton fault, still visible traversing the section exposed 

 under the drift at Craiglockhart Station despite protective 

 brickwork recently erected, and the Pentland fault on the 

 south-east. The amount of displacement involved was stupen- 

 dous. The strata beyond what may be called the buffer fold 

 south-east of the post-Carboniferous Pentland axis are found 

 disposed in a deep narrow syncline, with a maximum down- 

 throw exceeding ten thousand feet. 



Folds and faults reproduce themselves in the same places, 

 and the associated phenomena also. Exposed to attack on 

 both flanks, the Carboniferous rock-covering was gradually 

 worn away from the surface of the Pentlands as the area 

 on both sides subsided, denudation, as before, following crustal 

 displacement, removing superficial evidences of discordance. 

 Fault scraps disappear almost as soon as they are formed. 



The photograph (Plate IV.) of the Edge Coal strata rising 



