538 ltEPOux — 1884. 



series, which near the contact is converted into a highly micaceous 

 rock, the so-called gneiss of Brazil Wood. 1 



(/3) Some large masses of a coarse syenite on the south and south- 

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

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

 analysis, appears to be intermediate between a syenite and diorite. 



(y) Some smaller masses of a rock less coarsely crystalline, rather more 

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

 Forest. Notwithstanding some chemical difference, there seems good 

 reasons for considering these two groups of intrusive rocks to be closely 

 connected. 



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

 diabases, and, at Mountsorrel, a compact felsite cutting the hornblendic 

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



The rocks of tbe Forest area are probably prolonged undei'ground 

 beneath the Trias and Coal-measures for 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 Trias in the neighbourhood of 

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

 occurs at Enderby, about five miles south of Groby, and the most distant is 

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

 are igneous, but at Enderby 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 Narborough we have a boss which 

 might almost be called a quartz-felsite. 



(B) Walks. 



(8.) Pembrokeshire. — The region of Pembrokeshire about St. David's 

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

 of these was asserted by Dr. Hicks in 1871, 2 and their petrology has been 

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

 developed. 3 These may be thus summarised : that the base of the 

 Cambrian series in this part of Pembrokeshire (where it has now proved to 

 be fossiliferous) is marked by a conglomerate, in which pebbles of quartz, 

 quartzite, and felstone are present 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 ' halleflintas ' — i.e., silicious rocks of dubious origin — 

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

 base of this comes a granitoid rock, 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.' The correctness of these views was impugned, in the year 

 1883, by the present Director-General of the Geological Survey, Dr. A. 

 Geikie, 4 whose views may be thus briefly summarised : — 



1 Geol. Mag., Dec. ii., vol. vi. p. 481. 



- Harkness and Hicks, Q. J. G. S., vol. xxvii. p. 384. 



3 Q. J. G. S., vol. xxxi. p. 167; xxxiii. p. 22'.); xxxiv. p. 153 : xxxv. p. 285. For 

 other references, see Geikie, vol. xxxix. p. 261. 



4 Q. J. G. S., vol. xxxix. p. 261. 



