i9i4- I 9 I 5-] Blackfora Hill and its Agates. 163 



as it descends gently to the east and merging imperceptibly in 

 the plain. Viewed from the path below the Braid pond, the 

 hill resembles a broken table-land, its massive rock-face of 

 andesite, evidently once continuous over the Braids, left in air. 

 Its northern side presents a steep grassy slope descending from 

 a graceful ridge dotted with clumps of gorse, and manifesting 

 still the parallel drainage courses of a long-abandoned agri- 

 culture. 



The mass of the hill, if we trust the testimony of the quarry, 

 is hewn out of a single lava-flow, the lower limit of which is 

 determined by a bed of volcanic ash seen on the path-side be- 

 yond the western end of the artificial pond, the upper limit 

 by a basalt of the well-known ' Carnethy porphyry ' type, found 

 overlying the andesite at the Harrison Arch. The entire mass 

 is apparently tilted to the north-east, the external outline of 

 the hill not distantly reflecting its structure. Blackford Hill 

 has long been recognised as the northern termination of the 

 Pentland massif, to which its rock types belong. Eegarding 

 the origin of the Pentland volcanic rocks, various theories have 

 been advanced, a few of which we may notice. 



A horizontal section across the country from the neighbour- 

 hood of Auchterarder to Eoslin reveals what might be regarded 

 as a great anticline, with the volcanic rocks of the Ochil Hills 

 forming the north-western limb and the volcanic rocks of the 

 Pentlands the south-eastern limb, the core of the anticline 

 having collapsed, leaving its truncated ends pitching towards one 

 another across what is now the central valley of Scotland. A 

 consideration of this structure suggested a great volcano in the 

 intermediate distance, from which might have flowed the lavas 

 forming the two groups, and the neighbourhood of Linlithgow 

 was at one time regarded as the probable site of the volcano. 



Maclaren, in his ' Geology of Fife and the Lothians,' favour- 

 ing a local origin, supposed the eruptive centre to have been 

 north-east of the range, and that the lavas flowed in an elon- 

 gated hollow towards the south-west. In his 'Ancient Volcanoes 

 of Great Britain,' Sir Archibald Geikie also looked upon the 

 range as of local origin, and in that work put forward the view 

 that the Braid Hills mark the site of the principal vent of the 

 Pentland volcano, basing his opinion on the resemblance of the 

 decomposed and shattered rocks there to fine tuffs such as 



