88 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



whole of the visible necks of the Ochil Hills may be regarded as 

 one connected group, subsidiary to the main orifices which lay 

 rather farther to the south and west. 



For some miles eastwards from the central Ochils an interval 

 occurs, marked by the presence of only a few small intrusive masses. 

 But as the broad anticline of the Pirth of Tay opens out and allows 

 the lower or pre-volcanic members of the Old Eed Sandstone to 

 appear at the surface, another group of bosses emerges from the lower 

 sandstones and flagstones. Some of these cover a considerable space 

 at the surface, though a portion of their visible area may be due to 

 lateral extravasation from adjacent pipes, the true dimensions of 

 which are thereby obscured. Some of the masses are undoubtedly 

 sills. In the case of Dundee Law we probably see both the pipe and 

 the sill which proceeded from it ; the prominent well-defined hill 

 marking the former, while the band of rock which stretches from it 

 south-westwards to the shore belongs to the latter. The material 

 that forms the rocks and sills in this district is generally a dark 

 compact andesite. The rock of Dundee Law, Dr. Hatch remarks, 

 shows uuder the microscope " striped lath-shaped felspars abundantly 

 imbedded in a minute granular groundmass, speckled with granules of 

 magnetite, but showing no unaltered ferro-magnesian constituents.'^ 



Eeyoud this group of rocks no similar group is to be found 

 throughout the rest of the volcanic district. The rapid increase of 

 the porphyrites towards Montrose, however, points to the existence 

 of some vents in that direction, probably now concealed under the 

 waters of the North Sea. 



The sites of some of the orifices of discharge along the southern 

 line of volcanoes can be satisfactorily distinguished in the broken 

 hilly ground that flanks the Southern Uplands, from Carrick, in 

 Ayrshire, into the heart of Midlothian. Beginning at the south- 

 western end, we find a number of large bosses of pink felsite rising 

 through the Silurian rocks and the lower parts of the Old Red 

 Sandstone ; while around them, broken by faults and much cut down- 

 by denudation, lie the bedded porphyrites and volcanic conglo- 

 merates. Some of the detached eminences of erupted rock that rise 

 through the Old Eed Sandstone between these hills and the adja- 

 cent coast not improbably mark the sites of other vents. The con- 

 spicuous Hill of Mochrum, near Maybole, may be one of these, but 

 it difi*ers from the rest in composition, for it consists mainly of tufi^ 

 and ashy conglomerates, with a mass of porphyrite, part of which 

 IS amygdaloidal. 



Some forty miles to the north-east, the conspicuous and graceful 



