366 Geological Society : — 



survey which he had made of the district with Messrs. Peach and 

 Topley, and of his study of a series of more than 100 thin slices of 

 the rocks collected at St. David's. He found that he could corro- 

 borate generally the descriptions of previous writers on the micro- 

 scopic structure of the rocks, and that investigation with the micro- 

 scope amply confirmed the deductions he had drawn from observa- 

 tions in the field. 



1. Order of Succession of the Rocks. — The following rock-groups 

 in the Lower Cambrian series are recognizable at St. David's, and 

 are given in descending order : — 



4. Purple and greenish grits, sandstones, and shales. 



3. Green and red shales and sandstones, with true tuffs 



(Lingulella primceva). 

 2. Quartz conglomerate. 

 1. Yolcanic group (tuffs, schists, lavas). 



The volcanic group forms the oldest part of the Cambrian series 

 at this locality. The bottom is not reached ; but about 1800 feet 

 are visible. It consists mainly of purplish-red, green, grey, and pale 

 tuffs, with occasional breccias and bands of olivine-diabase. Analyses 

 of some of these rocks had been made for the author by M. Eenard, 

 of Brussels, and Mr. J. S. Grant Wilson, of the Geological Survey 

 of Scotland. The tuffs are partly basic, derived from the disruption 

 of diabase lavas (48 per cent, of silica), partly acid, from the destruc- 

 tion of fine felsites (72-80 per cent, of silica). The microscopic 

 structure of the tuffs was described, and slides and drawings were 

 exhibited. The lavas are varieties of olivine-diabase. Their augite 

 is remarkably abundant and fresh ; and they contain scattered larger, 

 well-formed, as well as imperfect crystals of olivine, generally in the 

 form of hsematitic pseudomorphs. No instance was observed of a 

 siliceous lava having been erupted at the surface. The felsitic frag- 

 ments in the tuffs must have been derived from the explosion of 

 lavas that do not seem to have flowed out above ground. It was 

 pointed out that this fact is exactly paralleled in the case of the 

 volcanic group of the Lower Old Ped Sandstone in the Pentland 

 Hills. 



In relation to the quartz-conglomerate, allusion was made to the 

 constant recurrence of such conglomerates in the series of geological 

 formations, and to the fact that they do not necessarily mark uncon- 

 formability or the natural base of groups of sedimentary rocks. 



2. Geological Structure of the District. — It was shown that the 

 rocks have been folded into an isocline or inverted anticline, so that 

 in one half of the plication the dip of the strata is reversed. 



The groups above mentioned are found in their proper order on 

 both sides of the axis which runs through the volcanic group. The 

 granite has risen irregularly through the eastern limb of the iso- 

 cline. Small faults may occur here and there along the edge of the 

 granite, but they do not in any way affect the general structure. 



3. The Foliation of the District. — There has been extensively 

 developed at St. David's a fine foliation of particular kinds of rock, 



