98 



THE GEOLOGIST. 



moved, a quantity corresponding to jive times the existing mass of 

 the Pentlands, and as these hills are 14 miles long by 3 broad, that 

 mass must have been equal to 42 cubic miles. 



This great denudation must evidently have required a long lapse 

 of ages for its completion. While Permian and Secondary rocks 

 were being deposited in other parts of the island, as, for instance, 

 Oolitic rocks on the east and west coasts of Scotland, north of the 

 Grampians, and Permian and Trias in the south, the central part of 

 Scotland was undergoing a great change. No strata apparently were 

 being elaborated, many however were being swept away ; the work of 

 abrasion and demolition was everywhere dominant. The softer strata 

 of sandstone, shale, and limestone were carried off, and the nume- 

 rous trap-rocks of the district left standing out in high relief, till in 

 short the country began to assume very nearly its present appearance. 

 The first period of denudation then is ended, and another period 

 of volcanic activity is at hand. 



Second Period. — Midlothian, still under water, is subjected to the 

 ravages of a submarine volcano. The Tertiary period has come, and 

 the igneous forces round Edinburgh, so long quiescent, so many 

 millions of years at rest, which had not disturbed the neighbourhood 

 since the Lower Carboniferous age, again become vigorous, again 

 break out on the site of the old volcano. One can almost fancy the 

 scene. The sky begins to lour, a rumbling noise is heard, and the 

 crater of Arthur's Seat sends forth from its mouth showers of scoriae 

 and volcanic ash. As it fell over the truncated edges of the older 

 rocks of the hill, large fragments of the sandstones, greenstones, ba- 

 salts, porphyries, and amygdaloids were embedded in the felspathic 

 paste, and soon converted into conglomerate when cooled. Then the 

 crater of the summit was plugged up with solid basalt, and the volcanic 

 material not being able to find a vent, burst out through a lateral 

 orifice and formed what is now the Lion's Haunch. Last of all, an- 

 other ejection of lava took place and wrapping round the north- 

 east end of the basalt gave rise to the felstone of this part of the hill, 

 and closed for ever the eruptions of Arthur's Seat (PI. V. Pig. 3). 



The hill however does not present the same appearance to-day that 

 it did then. The ash must have fallen thickly over the site of Edin- 

 burgh, as is evident from the frag mental condition of the cliff of ash 

 above Hunter's Bog, and also of the basalt on the slope at the south 

 side of the hill, which never could have stopped abruptly there. Conse- 

 quently we are led to conclude that Midlothian has been subjected to 

 another process of denudation, subsequent to the ejection o\ these 

 rocks. What was the nature of this denudation ? It cannot have 

 been due to atmospheric causes, nor to the action of the waves of 

 the sea. Its true cause must be sought for in the Glacial period, 

 when another change of contour was effected on Arthur's Seat. The 

 greenstones and basalts were ground down and polished by the fric- 

 tion of the moving ice, the loose ash was swept away, and only that 

 part of it left which had been more firmly compacted round the 

 heated orifice of the summit (PI. V. Pig. 4). 



