94 THE GEOLOGIST. 



sented by the Secondary formations is involved in darkness. On the 

 east and west coasts of Scotland rocks of Secondary age occur north 

 of the Grampians, but of the physical history of Central Scotland 

 during the time these rocks were deposited, we know nothing. From 

 the time of the deposition of our Upper Coal-beds, or, it may be, of 

 some Permian Sandstones, up till the time when the whole island 

 was locked fast in one immense mantle of ice, we are almost entirely 

 ignorant of what was going on in that part of the country which lies 

 between the Grampians and the Forth. And the man who shall de- 

 cipher for us the physical geography of that period, and reveal to us 

 the old surface of that district, with its vegetation and animal pro- 

 ductions, prior to the time of the Boulder Clay, will have rendered 

 no small service to the cause of Scottish geology. 



But although we have not as yet been able to trace the old surface 

 of the land, we are not altogether without data to guide us in our 

 researches. One thing is clear and certain, — a great change was 

 taking place over the whole face of this region. 



Both during and after the deposition of the coal-measures, great 

 volcanic agencies were at work. In the neighbourhood of Edinburgh 

 they were particularly active. They had been so during the Old Red 

 Sandstone period, and contributed in no small degree to the forma- 

 tion of the Pentland Hills. But towards the close of the Old Eed 

 period and the commencement of that time when our Lower Carbo- 

 niferous Sandstones were being formed, the Pentland ridge began to 

 sink under those Carboniferous seas. The crater of eruption, which 

 had been so active in the Pentland area, and produced that varied 

 mass of material now presented to us as sheets of felstone and ashy 

 bands, became gradually quiescent, till at last it was entirely covered 

 over with the increasing deposits. But, while these deposits were 

 being accumulated, the igneous forces broke out again at Arthur's 

 Seat. Most of the rocks which now form the eastern part of the hill 

 belong to this period. The greenstone of the Long Row, the red 

 nodular ash and the thin bands of red ashy sandstone to be seen in a 

 section at the south side of the Queen's Drive, the green felspathic 

 ash of the Dry Dam, the black columnar basalt above it (which also 

 constitutes the crag on which stands St. Anthony's Chapel), the two 

 basalts behind St. Anthony's, and which form the lower part of 

 Dunsapie Hill and Calton Hill, and lastly, that group of felstones 

 which form the remainder of the eastern part of the hill from the 

 top of Dunsapie across Whinny Hill to St. Margaret's Station, and 

 which make up most of the Calton Hill, — all these igneous productions 

 were contemporaneous with the Lower Carboniferous strata, and are 

 found interbedded with them. Consequently, all these traps are 

 subaqueous. Layers of sediment were first formed, then an eruption 

 of lava took place, which spread itself in sheets over the strata ; 

 then more layers of mud and sand were deposited by the ocean, and 

 then more sheets of lava ejected by the igneous agency below. It 

 is evident, moreover, that these traps must have been more or less 

 horizontal, for if they had been poured over an inclined surface, 



