no THE OLDER ROCKS AT ST. DAVIDS. 



By looking through the last few volumes of the Quarterly 

 Journal of Geol. Society, a series of papers will be noticed on 

 various fossiliferous members of. the Cambrian series- The faunae 

 have been described in turn by Dr. Hicks from the Arenig, the 

 Tremadoc, Lingula beds, Menevian, and so forth down into the 

 lowest Cambrian, new forms being discovered where none were 

 known, and life being pushed back farther and farther into the 

 past. Below the Conocoryplie Lyellii beds, below the red shales 

 with LingulellaprimcBva — the oldest fossil in Great Britain — was still 

 a o-reat thickness of sandstones which has yielded, hitherto, no 

 fossils ; while at the base of the Cambrian lies the great conglomerate, 

 intensely hard, red in colour usually, and formed of rolled quartz 

 or quartzite pebbles mainly, most firmly cemented together. This 

 massive rock breaks off, at the out-crop, in blocks weighing several 

 tons usually 3 it constitutes, therefore, a well marked base to the 

 Cambrian. With these sedimentary and unaltered beds we have 

 not now to concern ourselves, but below all this Cambrian, and 

 partly concealed by them, come in the Pebidian and Dimetian 

 series — the Pre-Cambrian beds. 



I enjoyed the great advantage of being taken over a great part of 

 the o-round by Dr. Hicks himself, who most courteously showed 

 the most fossiliferous places in the sedimentary, and in the 

 crystalline series demonstrated the sections especially noticed in 

 his paper. In collecting, therefore, I knew that I was dealing 

 with the same objects referred to by Dr. Hicks. 



From the history of his researches given in his introduction to 

 his last memoir, we note that the idea of a Pre-Cambrian period 

 was enunciated in 1864 {he. cit. p. 229) in collaboration with 

 Salter. 



Immediately under the city of St. Davids is a ridge of crystalline 

 rocks— mapped as Syenite by the Geological Survey,-— this was the 

 Pre-Cambrian Island. It has been the subject of Dr. Hicks' 

 examination, from time to time, during the last fifteen years. 



His latest description of the Pre-Cambrian divides them into 



