i9i4- I 9 I 5-] Blackford Hill and its Agates. 1 69 



ravines and ridges. The glacial period was one, it is true, 

 of sterilisation, but it seems also to be true that it removed 

 violent inequalities of surface which no device of agriculture 

 could have ameliorated, and left, despite a legacy of marshes, 

 lochs, and erratic boulders, a surface relatively ideal for suc- 

 cessful cultivation. 



The Vein Agates. 



A visit to the quarry at Blackford Hill shows that the rock 

 has been extensively fissured and faulted. The faults are 

 faults of compression, lateral thrust faults, trending generally 

 north-west and south-east. The fault cheeks usually carry 

 a deposit of calcite formed during movement, often exhibiting 

 many superimposed slickensided layers. The fissures, mostly 

 older than the faults, traverse the rock in all directions, 

 and their parted surfaces have been re-cemented by silica, 

 giving rise to jasperous veins, the so-called Blackford Hill 

 Agates. 



Agates, properly so called, are ■formed, not in veins, but in 

 the steam cavities of andesite lavas. Within these cavities 

 films of silica, varied in colour, have been precipitated from 

 solution, the vesicle having been gradually filled up from 

 circumference to centre. "We need not here discuss the 

 various theories advanced to account for the ingress of the 

 silicious material. Goodchild remarks ('Scottish Agates') 

 that no second layer of opal inclined to a first layer has 

 been found in Scottish Agates, and that since the opal layer 

 is laid down horizontally in the cavity, therefore the opal of 

 the Scottish Agates was deposited before any disturbance of 

 the rocks took place. Further, he affirms that the Upper 

 Old Eed Sandstone contains specimens of agates which have 

 certainly come from the andesite lavas of Lower Old Bed 

 Sandstone age, from which he concludes that the Scottish Old 

 Bed vesicular Agates were all formed before Upper Old Bed 

 Sandstone time, indeed before the earth-movements which 

 took place between the Lower and the Upper Old Bed. No 

 inference of this nature can be made regarding vein agates, 

 for the vein, unlike the vesicle, is not restricted to compart- 

 ments which were in existence before the lava cooled. Sub- 



