Geological Society of London. a 20m 
the Lower Cambrian series are recognizable at St. David’s, and are 
given in descending order :— 
4, Purple and greenish grits, sandstone, and shales. 
8. Green and red shales and sandstones, with true tuffs 
(Lingulella primeva). | 
2. Quartz conglomerate. 
1. Volcanic 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 tufts, 
with occasional breccias and bands of olivine-diabase. Analyses of 
some of these rocks had been made for the author by M. Renard, 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 destruction 
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 hematitic 
pseudomorphs. No instance was observed of a siliceous lava having 
been erupted at the surface. The felsitic fragments 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 Red 
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 isocline. 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, 
more especially of certain fine tuffs and shales, which have passed into 
the condition of fine silky unctuous hydro-mica-schists or sericite- 
schists. A series of microscopic slices was described which showed 
that the original clastic structure of the beds remains quite distinct, 
though an abundant development of fine flakes of a hydrous mica has 
taken place. This structure more particularly characterizes the fine 
parts of the volcanic group, but it occurs also on various horizons in 
the groups above the conglomerate, thus linking the whole as one great 
continuous series of deposits. The author connected it with the plica- 
tion of the district, and pointed out the great interest attaching to 
these fine schistose bands as revealing some of the incipient stages of 
