428 'THE LLANDOVERY ROCKS OF MONTGOMERYSHIRE. [Aug. I9II. 



upon a thin stratum of pale ashy shales. About 50 feet of beds are 

 -exposed in the quarry, giving the following section (fig. 3) : — 



•Eig. 3. — Section near the top of the Gwern-y-brain Valley. 



r 



tPine, hard, blue 

 ' fossiliferous "S 

 ■ inudstone. 



iUard, blue, cal- 

 careous beds 

 with shaly 



, partings. Each J ~- .v -_l--i--l-^ p 



bed is overlain "\ \ 



by a band of 

 -coarse fels- 



pathicgrit. 



• Pale-grey phos- 

 phatic and 



ashy shales. 



Thickness 

 in l'eet 



2D 



12 



The lower shale-band is 

 phosphatic. The calcareous 

 beds in the quarry are thick 

 and massive. They are, as 

 a rule, fine-grained, blue, and 

 hard. Each thick bed has 

 a curious structure, and it 

 gradually passes upwards into 

 a band of coarse whitish grit. 

 A microscopic examination 

 showed that each of these 

 grit-bands was exceedingly 

 rich in large angular frag- 

 ments of remarkably fresh 

 plagioclase-felspars, along 

 with quartz-grains, sometimes a good deal of mica, and granules 

 -of fine-grained igneous rocks, of a basaltic or andesitic nature. 

 Some of the igneous fragments are probably ashes. The fresh 

 "felspars and the andesitic fragments seem to indicate rapid denuda- 

 tion in a neighbouring volcanic area. A faulted monoclinai fold 

 ".is seen in the eastern corner of the quarry ; the throw is about 

 10 feet to the west. A second fault is seen on the west side of 

 the quarry, throwing down again to the west ; but the amount 

 <eould not be measured. 



The fossils, which are beautifully preserved, and often filled with 

 milk-white calcite, correlate these upper beds very closely with the 

 horizon of the Lower Bala Limestones both of North Wales and of 

 South Wales, while the presence of ashy grits in them shows their 

 intermediate character. The group is sharply defined on palaeonto- 

 logical grounds. (See List on p. 429.) 



(4) Gwern-y-brain Group. — This consists of a series of thin, 

 fine-grained, jet-black shales, which readily splinter to fragments. 

 Near the base in Gwern-y-brain occurs a band of black crystalline 

 limestone. These beds are only fossiliferous at certain horizons, 

 and occur over a limited area. They are seen above the Gaer- 

 fawr Limestone in Gwern-y-brain, and can be traced thence to 

 the dingle below Trawscoed Hall. Here the outcrop seems to 

 be shifted by a small fault ; but shales can be traced along a 

 depression towards Cross Wood, where they are overlapped by 

 the Powis-Castle Series. The beds are crowded with inarticulate 

 brachiopods and ostracods at certain horizons. Gasteropods also 

 are very abundant ; while articulate brachiopods, where present, 

 are characteristically dwarfed. 



