Professor Geikie on the " Pitclistone " of Eskdale. 235 



together and founded the Wernerian Society, of which he 

 was first President, and, whilst it existed, its main buttress. 

 At one of the earliest meetings of this institution he pro- 

 pounded a series of mineralogical queries, among which we 

 find the following — "What are the extent and particular 

 geognostic relations of the black pitclistone of Eskdalemuir, 

 in Dumfriesshire ? " 



This query, so far as I have been able to discover, received 

 no further elucidation from Jameson himself, and has not 

 been regarded by any subsequent investigator. The answer 

 which now can be given to it will be in some measure an 

 indication of the progress of petrography during nearly three- 

 quarters of a century. 



Part II. — Stratigraphical. 



The first point to be noticed refers to the name bestowed 

 by Jameson upon the substance he described. The Wer- 

 nerians, being especially proud of their mineralogy, seldom 

 lost an opportunity of pointing out the defectiveness of that 

 of their Vulcanist opponents. It is curious, however, to find 

 that Jameson was himself in error when he identified this 

 Eskdale rock as pitchstone. He states that the rock passes 

 into basalt. But true pitchstone, such as that with which 

 he was familiar in Arran and elsewhere, contains some- 

 where about 65 per cent, of silica, while in basalt the pro- 

 portion of this oxide is not more than from 45 to 55 per 

 cent. It is incredible, therefore, that crystalline basic basalt 

 can pass in the same little hill into vitreous acid pitchstone. 

 Some years ago, being confident that the rock could be no 

 other than one of the vitreous conditions of basalt which I 

 have found in many basalt dykes both on the mainland and 

 among the Western Islands, I made an excursion into Esk- 

 dalemuir, and found that this was undoubtedly the case. 



Another and more important mistake was committed by 

 Jameson in his determination of the geological structure of 

 the locality. He says that the hill- tops are covered with 

 pitchstone, which overlies the much-inclined strata of grey- 

 wacke. N^ow, in reality, the vitreous rock forms only a part 



