90 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Survey, I found it extremely difficult to understand. Being then 

 myself but a beginner in geology, and the study of old volcanic 

 rocks not having yet advanced much beyond its eleraentary stage, I 

 failed to disentangle the puzzle. Not until after more than twenty 

 years, largely spent in the investigation of Volcanic rocks elsewhere, 

 had I an opportunity of re-surveying the ground and bringing to 

 its renewed study a wider knowledge of the subject. I will here 

 offer an outline of the chief results, hitherto unpublished, at which 

 I have arrived. 



The abrupt truncation of the bedded lavas and tuffs along the 

 base of the northern face of the Pentland Hills marks approximately 

 the southern margin of a large vent from which at least some, if 

 not most, of these rocks were probably ejected. The size of this 

 vent cannot be ascertained on account of the unconformable over- 

 spread of Lower Carboniferous strata. But that it must have been 

 a large and important volcanic orifice may be inferred from the 

 fact that the visible area of the materials that fill it up measures 

 two miles from north-east to south-west, and a mile and a half 

 from south-east to north-west, thus including a space of rather 

 more than two square miles. Its original limits towards the north 

 and south can be traced by help of the bedded lavas that partially 

 surround it, but on the two other sides they are concealed by the 

 younger formation. We shall probably not over-estimate the actual 

 area of the vent if we state it at about four square miles. 



The materials that now fill this important orifice consist mainly 

 of ' claystones :' that is, dull rocks, meagre to the touch, varying in 

 texture from the rough porous aspect of a sinter through stages of 

 increasing firmness till they become almost felsitic, and ranging in 

 colour from a dark purple-red, through shades of lilac and yellow, 

 to nearly white, but often strikingly mottled. A more or less 

 laminar structure is often to be observed among them, indicating 

 a dip in various directions (but especially towards the north) 

 and at considerable angles. Throughout this exceedingly fine- 

 grained material lines of small lapilli may occasionally be detected, 

 also bands of breccia, consisting of broken-up tuff of the same 

 character, and of fine ' hornstone ' and felsite, with delicate flow- 

 structure. Though exhibiting on the whole so little structure, this 

 tract must be regarded as consisting mainly of fine volcanic dust- 

 derived from the explosion of lavas of a much more acid type than 

 those which flowed out to form the bedded porphyrites and diabases 

 of the Pentland Hills. Yarious veins, dykes, and small bosses of 



