ON ROCK-SPECIMENS FROM N.W. PEMBROKESHIRE. 357 



30. On some Rock-specimens collected by Dr. Hicks in N.W. Pem- 

 brokeshire. By T. G. BoNNEY, D.Sc, LL.D., F.R.S., Professor 

 of Geology in University College, London, and Follow of St. 

 John's College, Cambridge. (Read May 26, 1886.) 



The specimens collected by Dr. Hicks during his late visit to Pem- 

 brokeshire have a most important bearing on the question of the 

 existence or non-existence in that region of a considerable group of 

 Pre-Cambrian rocks. In addition to these, the slides described by 

 Mr. T. Davies in the ' Quarterly Journal ' for 1884 were placed in 

 my hands for purposes of comparison. I also possess a small series 

 cut from specimens either collected by myself on occasion of a short 

 visit in 1882 or given to me by Dr. Hicks. 



The main points at issue in regard to the geology of the district 

 are these : Do we find here an important group of rocks anterior to a 

 well-defined base of the Cambrian series ? or are certain granitoid 

 and felsitic rocks (the so-called Dimetian and Arvonian) only in- 

 trusive masses of later date than the lower Cambrian ? and is the so- 

 called Pebidian merely a record of volcanic eruptions which occurred 

 in the earliest days of the Cambrian period, no more deserving of 

 separation from it than are the volcanic rocks of the Arenig or Bala 

 from the other members of those formations ? Subordinate and 

 highly interesting questions are : What is the true nature of the 

 granitoid (Dimetian) and of the felsitic (Arvonian) rocks ? Are 

 they metamorphic rocks in the ordinary sense, or are they igneous ? 

 If the former view can be established, the controversy is practically 

 at an end ; but if the matter must remain in uncertainty, or even 

 if the rocks be proved in both cases to be igneous, they may yet be 

 Pre-Cambrian, and the question must be settled by stratigraphical 

 evidence. 



Cambrian Conglomerate of Chanter's Seat. 



The first group of specimens which I have to describe is from the 

 Cambrian conglomerate near Chanter's Seat. The matrix of this 

 conglomerate is greenish grey in colour, a little brown on slightly 

 weathered surfaces. The appearance of the rock suggests that it is 

 a kind of arkose. A portion of it bears considerable resemblance 

 to parts of the Dimetian of St. David's and of the granitoid rocks 

 from Brawdy when locally crushed. It is crowded with specks of 

 light-coloured felspar, grains of quartz, and rather well-rounded 

 little pebbles, usually not exceeding ^ inch in diameter (about 0*2 

 inch is a common size) ; many of these are quartz, others evidently a 

 granitoid rock. After Mr. T. Davics's excellent description of speci- 

 mens from this locality and elsewhere (Quart. Journ. Geol. Soc. 

 vol. xl. p. 548), I need not enter into minute details. 



The slides all abound in quartz-grains, sometimes well rounded, 

 many of which, in the nature and arrangement of the inclusions. 



