T. C. Cantrill — Coal-boring at Presteign, Radnorshire. 489 



Limestone in the openwork, as already described (p. 486). The 

 thickness of the limestone itself, although amounting to 20ft. Sin. 

 as cut by the boring, would, if measured at right augles to the 

 bedding, be somewhat less, but could scarcely be reduced to 12 feet, 

 the maximum space permissible in the openwork. It is possible, 

 therefore, that the thickness at the outcrop has been reduced as the 

 result of squeezing. The balls of limestone in Item 7 show that the 

 conditions that preceded the deposition of crystalline limestone were 

 similar to those which followed. The boring agrees with the 

 surface exposures in showing the rapidity of the change from the 

 grits and conglomerates of the Upper Llandovery to the shales, 

 mudstones, and limestones of the Woolhope and Wenlock. 



Item 8 forms the top of the Upper Llandovery Sandstone, and the 

 white pebbles in the next item identify this bed with one of those 

 exposed in the openwork. The cores, however, yielded no recogniz- 

 able fossils. 



But while there is no doubt that the upper beds of Items 8-19 

 represent the Upper Llandovery Sandstone, there is reason to believe 

 that the lower of these beds are Longmyndian, and that a nucleus of 

 Archaean rocks, directly underlying the Upper Llandovery, runs 

 through the Corton ridge, pi-obably from one end to the other. This 

 suggestion is based on the following evidence. 



Professor E. J. Garwood, during a visit at Easter, 1915, obtained 

 from the site of the boring a sample of rock to which special interest 

 attaches, on account of its strong resemblance to the conglomerates 

 of the Bayston Group of the Longmyndian System. The rock is 

 represented by part of a 3 in. core, which, with this diameter, 

 must have come from the upper part of the boring. In the hand- 

 specimen the rock is a pebbly tough grit, of a purple and green 

 colour, composed of scattered subangular pebbles (up to If inches, 

 the majority being about half an inch in length) of purplish-red 

 .quartzite approaching jasper in appearance, with others of white 

 quartzite, set in a greenish-grey matrix of small quartz-grains, 

 small greenish pebbles, and a greenish interstitial paste. 



Under the microscope a slice of the rock (E. 1 1237) x shows that 

 while many of the small quartz-grains are angular, others are rounded, 

 and, as Dr. H. H. Thomas has suggested to me, appear to have been 

 derived from the detrition of a quartz-porphyry. This is supported 

 by the fact that one of the quartz-grains is partly surrounded by 

 adherent glass. A few grains, about the size of the smaller quartz- 

 grains, are of silicified banded rhyolite. The small green pebbles 

 appear to be chloritic sediments ; the interstitial paste seems to 

 consist of chlorite and quartz, with limonitic iron oxide. The rock 

 contains neither felspar nor calcareous matter. 



A slice (E. 11236) 1 taken from one of the large pebbles of purplish- 

 red quartzite conspicuous in the hand-specimen shows that the 

 quartzite is thoroughly silicified, fine-grained, and contains many 

 skeleton rhombohedra after some slightly ferruginous rhombohedral 



1 These numbers refer to the registered rock-slides in the collection at the 

 Geological Survey Office, Jermyn Street, London. 



