Vol. 68.] PKE-CAMBRIAN AND CAMBKIAN OF PEMBEOKESHIEE. 391 



The rocks are described under the following headings : — 



Thickness 

 in feet. 



T-, -o f 7. Banded shales Unknown. 



FoEDBEDS...|g Muslandarit Abgut 100 



f5. Grreen quartzitic sandstones More than 100 



„. Tx I 4. Purple sandstones About 250 



Welsh Hook I 3 ^^,{,i-,^i,, 50 



±JEDS. I 2_ Green felspathic sandstones 450 



( 1. Conglomerate, pebbly grit, and quartzite to 150 



The lower beds only, corresponding to the numbers 1-4, are found 

 in the south-western area ; but, unless otherwise stated, the obser- 

 vations on the Welsh Hook sequence may be assumed to apply to 

 the remaining areas. 



The Welsh Hook Beds. 



(1) The basal beds vary in character from conglomerate to pebbly 

 grit or coarse quartzite in a comparatively short distance. 



At Stone Hall Mill a coarse conglomerate, with pebbles of liver- 

 coloured quartzite, rests upon highly-sheared rhyolitic ashes ; while 

 to the east of Stone Hall the basal beds are saccharoidal pebbly grits 

 containing pale pink quartz-pebbles, probably derived from certain 

 types of the quartz-porphyry which contain amethystine quartz : 

 in this locality the grit appears to overlie a mass of quartz- 

 porphyry. 



The most remarkable development of the conglomerate is that in 

 the south-western area, to the east of Silver Hill. Unfortunately 

 there is no exposure ; but, during our visits, some of the fields, 

 intervening between the outcrop of the green sandstone (2) and 

 the granite, were covered with big blocks of coarse conglomerate 

 derived from the immediate subsoil. The large lumps contain 

 pebbles of vein-quartz, granite, and diorite ; while numerous well- 

 rounded pieces of vein-quartz, diorite, granite, porphyry, and 

 quartzite, occurring in association with them, are almost certainly 

 pebbles weathered out from the conglomerate. The presence of the 

 Knaveston dioritic mass about 400 yards away might account for 

 the diorite-pebbles. This conglomerate was assayed for gold by 

 Dr. W. Pollard, of H.M. Geological Survey, but it yielded no values. 



There is a small quarry in conglomerate, with pebbles up to 

 4 inches in diameter, on the western side of Brandy Brook, half 

 a mile north of Gignog ; all the pebbles observed there were of 

 ■quartzite and vein-quartz. 



On Tre-rhos Moor, and to the south, the basal beds have disin- 

 tegrated to some depth ; at the former locality they have been dug 

 for sand, and in other places the fields are covered with well-rounded 

 pebbles. 



(2) The succeeding group is made up of greenish-blue, felspathic, 

 somewhat micaceous or chloritic sandstones in their fresh state ; 

 but they usually weather to soft, buff-coloured or pale yellowish 



Q. J. G. S. 1^0. 271. 2 P 



