i9i4 _I 9 I 5-] Blackford Hill and its Agates. 165 



displacement, tending always to obliterate superficial evidences 

 of discordance. Below Torphin Crags, where the Pentland 

 Hills are nearest to the Colinton fault line, only a stump of 

 the lowest known lava of the series remained at the close of 

 this period of denudation, a thickness of possibly 5000 feet 

 having been swept from this particular area, for the succeed- 

 ing deposits of Upper Old Eed Sandstone, largely derived from 

 the wearing down of the lavas, are found resting here on its 

 eroded surface. These operations, and the structure resulting 

 from them, may be realised by consideration of a rough 

 diagrammatic model. A dozen sheets of thick card-board 

 glued together, bent in the form of an elliptic, very low 

 arched dome, with one half brokeu off along its longer axis, 

 illustrates the fault exposing the outcrops to attack ; and a 

 horizontal cut through the remaining half exhibits the effect 

 of prolonged denudation laying bare successively areas of 

 every member of the series. 



Over these exposed areas the waters of the Upper Old Eed 

 Sandstone lake rose, and laid down their sediments. A lake 

 at the level of the present 500-feet contour line would not 

 unfaithfully represent the scene in view. The Pentlands, at 

 that time, including Blackford Hill and the Braids, formed a 

 diminishing island in the midst of the lake. 



It was a remarkable world that presented itself in our area 

 at the time of the Upper Old Eed Sandstone, a world the 

 description of which would have beggared the pen of the 

 author of 'The Ancient Mariner': a desert land with meagre 

 and primitive vegetation ; a world without flowers ; a land 

 fauna of invertebrates ; no bird beat the air or alighted on its 

 waters ; its lakes were inhabited by primitive types of fish, 

 leading a precarious existence in the presence of an astonish- 

 ingly restricted food supply and recurrent dessication. One 

 of the most noteworthy of palseontological enigmas, the Dura 

 Den episode, belongs to this period, productive of much 

 speculation, and to which, until recently, no satisfactory 

 solution was offered. Here, in one of the rock layers, fossils, 

 mainly of Holoptychius, were found densely crowded together. 

 What disaster had overtaken them in the mysterious lake of 

 that far-off time, removed from us, in Mr Goodchild's generous 

 estimate, by upwards of four hundred million years ? Was it 



