98 
THE GEOLOGIST. 
moved, a qnautltv correspondiDg to jive times the existing mass of 
the Pentlands, aiad 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 ; tlie 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 jyresent appearance. 
The first period of denudation then is ended, and another period 
of volcanic activity is at hand. 
iiiecond 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 em.bedded 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. Fig. 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 Vae 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 of these 
rocks. What was the nature of tJiis 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 eff'ected 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 summ.it (PL V. Fig. 4). 
