14 THE JOURNAL OF GEOLOGY. 



Scotland remains to be determined. The north-western part of 

 the Longmynd ridge is made of red sandstones and conglomer- 

 ates, which certainly resemble the Torridonian rocks of Ross and 

 Sutherland. 



At the base of the Cambrian rocks in Wales, Dr. Hicks has 

 described a marked volcanic series under the name of "Pebidian," 

 which he claims as pre-Cambrian, alleging that it is separated 

 from the Cambrian system by an unconformability, and a band of 

 conglomerates. I have carefully studied the evidence on this 

 ground, and have come to the conclusion that there is no 

 unconformability at the line in question, but that the ordinary 

 Cambrian strata graduate downwards into the volcanic group and 

 can not be disjoined from it. I therefore regard the so-called 

 "Pebidian" as merely marking the duration of a volcanic period 

 in early Cambrian time. 



It will thus be seen that according to my view the unmistak- 

 ably pre-Cambrian rocks of Britain consist of, first and oldest, 

 the Lewisian gneiss ; second, the Torridonian sandstones and 

 conglomerates. The Uriconian and Longmyndian formations 

 may prove to be in part or in whole equivalents of the Torri- 

 donian. The Dalradian rocks have not yet had their position 

 determined. They may possibly mark a distinct pre-Cambrian 

 series, but it seems quite as probable that they are only a 

 metamorphic complex in which Archaean, Torridonian and 

 Cambrian, or even Lower Silurian rocks are included. 



Sir Archibald Geikie, 



Director-General of the Geological Survey of Great Britain and Ireland. 



