■-, 



i,l 



Hin 





538 



i!i;i'oia' — 1884. 



serifis, which noiir the contact is convm-tcil into ii highly micaceous 

 rock, the so-callod gneiss of Brazil Wood.' 



(P) Some liirgo masses of a coarse syenite on the south and soi'th- 

 western part of tlie Forest, also found to be intrusive in the upper portion 

 of the Charnwood series. The rock, from both microscopic and chemical 

 analysis, appeai-s to bo intermediate between a syenite and diorito. 



(y) Some smaller masses of a rock les;^ coarsely crystalline, rather more 

 basic, and distinctly dioritic. occurring in tlio more northern parts of the 

 Forest. Xotwithstanding some chemical d'H'erencjo, tiiero seems good 

 roil sons for considering these two groups of intrusive rocks to bo closely 

 connected. 



(8) A varied series of dykes and small intrusive masses, dioritos, 

 diabases, and, at Mountsoi-rel, a compact fclsitc cutting the hornblendic 

 granite. As a ruin they occur only in the above igneous masses. 



The rocks of the Forest area are probably prolonged underground 

 beneath the Trias iind ('oal-measures t'or a considerable distance, since 

 they have been struck once or twice in borings, and a number of bosses 

 of crystalline rock crop out from the 1'rias in the neighbourhood of 

 Narhorough, to the south of the Forest. The most northern of these 

 occurs at Enderby, about five miles south of ( i roby, and the most distant is 

 about live miles from that in a south-westerly direction. All the bosses 

 arc igneous, but at P]nderby quarrying has shown one of them to be intrusive 

 in a slaty rock, having a general resemblance to the upper part of the 

 Forest series. The dominant rock is a quartz-syenite or quartz-diorite 

 (for it is really intermediate), but at Xarboi'ough we have a boss which 

 might almost be called a quartz-felsite. 



(B) Walks. 



(f^.) Veii(hrol,-eshi're. — The region of Pembrokeshire about St. David's 

 has become classic ground in the history of Archiean rocks. The presence 

 of these was asserted by Dr. Hicks in 1871," and their petrology has been 

 worked out in a series of papers in which his views were gradually 

 developed.'' These may be thus summarised : that the base of the 

 Cambrian series in this ])art of Pembrokeshire (where it has now pi'oved to 

 bo fos.siliferous) is marked by a conglomerate, in which ])ebbles of quartz, 

 ({uartzite, and felstone are ])resent in large but variable quantities. 

 Beneath this, and unconformably overlain by it, comes a series of argillites, 

 volcanic breccias, and schistose rocks, under which is another series of 

 quartz-felsites and ' hiilleflintas ' — I.e., silicious i^ocks of dubious origin— 

 perhaps in some cases sedimentary, in others compact felstones. At the 

 base of this comes a granitoid i-ock, which Dr. Hicks considered to be 

 associated with thin bands of chloritic schist and of an impure dolomite, 

 and to be non-igneous in origin. To this last group he gave the name 

 ' Dimetian,' to the middle one of ' Arvonian,' to the upper one of 

 ' Pebidian.' Tho correctness of these views was impugned, in the year 

 18^0, by the present Director-General of the Geological Survey, Dr. A. 

 Geikie,' whose views may be thus briefly summarised : — 



' Otol. Mag., Dec. ii., vol. vi. j). 481. 

 - Harknc'ss an<I Hicks, Q. J. 11. S., vol. xxvii. p. liSl. 

 ^ (J. J. a. S., vol. xx.xi. p. 1G7 ; xxxiii. p. 22! i : xxxiv. p. 

 other references, secGeikie, vol. xxxix. j). 2C1. 

 * Q. J. a. S., vul. xxxix. p. 261. 



l.j:j; ;cxxv. ji. 265. Fo! 



