88 REPORT 1871. 



against tlie Lower Old Eed Sandstone and its volcanic rocks. Another result has 

 been the extreme tilting of the strata, -whereby the Limestone series along the east 

 side of the fault has been thrown on end, and even in some parts bent back into a 

 reversed dip. Hence, while on one side of the axis the Limestone series is some- 

 times only a few hundi-ed yards distant from the Old Red Sandstone, on the oppo- 

 site or north-west side the distance is fully eleven mUes, the intervening space 

 being there occupied by endless undulations of the lower divisions of the Carboni- 

 ferous system. Hence, too, the MiUstone-giit and Coal-measiu-es come in along 

 the centre of the Midlothian basin a short way to the east of the Pentland axis ; 

 while on the west side they are not met with tiU we reach the borders of Stii-ling- 

 shire and Linlithgow. 



Another remarkable and readily observable feature is, that on the west side of 

 the Pentland ridge the Carboniferous formations, from almost theu- base up to the 

 top of the Carboniferous Limestone series, aboimd in contemporaneous volcanic 

 rocks ; while on the east side, beyond Edinburgh and Arthur's Seat, such rocks are 

 absent until we reach the Garltou Hills, to the north of Haddington, where they 

 reappear, but in a very different type from that which they exhibit to the west. 



Let us now pass in review the different geological formations which come into 

 the district around us, beginning with the oldest and ascending through the others 

 till we reach the superficial acciunulations, and mark, in conclusion, how far the 

 present surface-features are connected with geological structure. 



[The author then described the various geological formations of the district — 

 Silurian, Old Ked Sandstone, and Carboniferous — dwelling in particiUar upon the 

 history of volcanic action in that part of Scotland. On this subject he remai'ked : — ] 



Outline of the History of Volcanic Action aroimd Edinhirr/h. 



The oldest ^-olcanoes of this part of Scotland were those which, duiing the time 

 of the Lower Old Red Sandstone, poured out the great sheets of poi-ph\Tite and 

 the showers of tuff which now form the main mass of the range of the Pentland 

 HiUs. During the same long geological period volcanic action was rife, as we 

 have seen, along the whole of the broad midland valley of Scotland, since to that 

 time we must refer the origin of the Sidlaw and the Ocliil Hills, part of eastern 

 Berwickshire, and the long line of uplands stretching from the Pentland Hills 

 through Lanarkshire, and across Nithsdale, far into A^Tshire. 



Of volcanic action, during the remainder of the Old Red Sandstone period, there 

 is around Edinburgh no trace. But early in the following or Carboniferous period, 

 the volcano of Arthur's Seat and Calton Hill came into existence, and threw out 

 its tiny flows of basalt and porphyrite, and its showers of ashes. From that time 

 onwards, through nearly the whole of the interval occupied by the deposition of 

 the Carboniferous Limestone series, the district to the west of Edinbm-gh was 

 dotted over with small cones, usually of tuff, but sometimes emitting limited 

 currents of different basalt rocks, more especially in the space between Bathgate 

 and the Forth, where a long bank, chiefly formed of such lava-currents, was piled up 

 over and among the pools and shallows in which the limestones, sandstones, shales, 

 and coal-seams were accumulated. To the north, also, similar volcanic activity was 

 shown in the Fife tracts nearest tlie Forth ; while eastwards, between ILaddiiigton 

 and Dunbar, there \a\ a distinct volcanic focus, where gieat showers of red fel- 

 spatliic tuff and widespread sheets of porphyrite were ejected to form a bank over 

 which the Carboniferous Limestone series was at length tranquilly deposited. 



Volcanic activity seems to have died out here before tlie close" of the Carboni- 

 ferous Limestone period. It remained quiescent during the deposition of the Mill- 

 stone-grit and Coal-measures ; at least no trace of any contemporaneous igneous 

 ejection is found in any part of these formations. The'intrusive masses of various 

 basalt rocks, which here intersect the older half of the Carboniferous system, arc, 

 in all probability, of Lower Carboniferous date, connected with the eruptions of 

 the interbedded volcanic rocks. The next proofs of volcanic action in this neigh- 

 bourhood are furnished by the upper part of Arthur's Seat. At that locality we 

 discover that after more than SOOO feet of strata had been removed by denudation 

 from the Pentland anticliual fold so as to lay bare the old Lower Carboniferous 

 volcanic rocks of Edinburgh, a new focus of eruption was formed, from which 



