52 THE ORDNANCE GEOLOGICAL SURVEY CHAP, n 



field-work. * I therefore wish/ he added, to be fore 

 most in recognising the deserts of the labourers who 

 have obtained them, among whom the Director par 

 ticularly cites Mr. Ramsay, already so favourably 

 known to us by his geological map and model of the 

 Isle of Arran. 1 



The St. David s map was published in 1845, an ^ 

 after its appearance a sheet of horizontal sections was 

 prepared and issued, showing what was believed to be 

 the general structure of the ground. Unfortunately, 

 twelve years afterwards, in second editions of these 

 publications, while great improvements were made in 

 the general stratigraphy, the views originally formed 

 by Ramsay as to the nature of the St. David s rocks 

 were so modified, though confessedly with his own con 

 sent and co-operation, that the essentially accurate inter 

 pretation at first adopted disappeared. In later years 

 the truly volcanic nature of much of the fragmental 

 rocks in that district, which in the second edition of 

 the map became altered Cambrian, was re-discovered, 

 and the merit of the first observations was for a time 

 obscured. 2 



There was yet another feature in which Ramsay 

 improved the mapping of the Survey. He traced out, 

 where practicable, lithological subdivisions among the 

 older Palaeozoic rocks, which had not previously been 

 subdivided, and was thus able to detect their sequence 

 and the general structure of the ground over which 

 they extended. In particular, even in his first year s 

 work, he drew a line between the black and purple 

 slates which, though not put on the published map at 



1 Proc. Geol Soc. iv. (1843), p. 76. 



2 The details of this question will be found narrated in the Quarterly Journal 

 of the Geological Society, vol. xxxix. (1883), p. 263. 



