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MR. A. WADE ON THE LLANDOVERY AND [Aug. I9II. 



richly fossiliferous, and are bastard limestones 

 similar in all respects to those of the Caradoc 

 Grits of Hoar Edge. These become more cal- 

 careous towards the top, where they are well 

 exposed in two quarries which are now being 

 worked. Here they are fossiliferous blue-grey 

 beds — impure limestones — containing coarse 

 gritty bands which consisb largely of fragments 

 of volcanic origin. Several small strike-faults 

 are present in the neighbourhood of the 

 quarries. The whole of the sequence is here 

 not less than 1000 feet thick. 



Gwern-y-brain Group. — Above the 

 quarries, a series of jet-black beds cross the 

 stream. A hard bed, forming a little water- 

 fall, proves to be a band of black crystalline 

 limestone about 10 feet thick. Above this 

 comes 50 feet of black shivery shales. 



Powis-Castle Group. — The Red Conglo- 

 merates and Sandstones cover these beds. 

 Their apparent thickness is misleading, owing 

 to a series of small rolls. 



The Cloddiau Group of mudstones 

 follows, and these are covered by shales of 

 Wenlock age, but the succession is here 

 obscured by drift. 



Y. Detailed Description of the Subdivisions. 



(A) Ordovician Hocks. 



(1) Trilobite-Dingle Shales.— These 

 beds are the lowest exposed in the district 

 around Welshpool. They are best seen in a 

 small dingle west of the town, called by 

 Murchison Trilobite Dingle. At the southern 

 end of the dingle, purple and grey shales are 

 seen dipping almost due west. These beds 

 are very fine-grained and well-bedded, and 

 frequently show the presence of slickensides 

 and incipient strain-slip cleavage. Occasion- 

 ally the beds are nodular, and pass into bluish 

 micaceous and flaggy mudstones. The joints 

 are usually stained red and orange with iron- 

 oxide. The beds are much disturbed, owing to the proximity of the 

 Welshpool Dyke on the west. 



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