SILTJKIAN OP THE DEE VALLEY. 279 



the slopes of the Berwyns, south of Llandrillo. About 300 yards S.E. 

 of the farm of Blaenydre, which is about a mile south of Llandrillo, 

 occurs an ash, which varies from a fine rock with calcite to a coarse 

 rock containing fragments some of which are apparently andesitic. 

 Prof. Bonney has kindly examined this and other rocks micro- 

 scopically ; and his descriptions are given below*. 



One of the doleritic rocks is well seen on the side of Clochnant, 

 about a mile and a half E.S.E. of Blaenydre, where it is traceable 

 by a broken edge of outcrop t. 



The occurrence of the andesitic ash is of interest, pointing, as it 

 does, to a connexion with the Borrowdale volcanic series, as de- 

 scribed by the Eev. J. Clifton Ward (Mem. Geol. Survey, Lake 

 Dist. p. 13 et seqq.). 



3. The Middle Bala Beds. 



Although this period was one of great volcanic outbursts in the 

 Snowdon district, it appears to have been comparatively quiescent 

 in the Dee-valley area : consequently we here get a strong litho- 

 logical resemblance to the Lake-district beds of this horizon. The 

 Bala Limestone has been correlated with the Coniston Limestone, 

 but would seem to have been deposited during the earlier part of 

 the Coniston-Limestone period, as above it a series of shales occurs, 

 whereas similar shales are intercalated with the bands of limestone 

 in the Lake district. 



At this period we get the first signs of those migrations towards 

 the Lake-district area which occur at least twice in the later de- 

 posits. Though many of the fossils of the Bala Limestone are 

 identical with those of the Coniston Limestone, we find that two of 

 the commonest Trilobites of the former, PJiacops apiculatiis and 

 Trinucleus concentricus, as also the genus EcJmiosphoRrites, which is 

 very typical of this rock, are not found in the Coniston, but occur 

 abundantly in the overlying Ashgill Shales J. 



* [Rock 300 yds. S. E, of Blaenydre. This seems to be a true ash, i. e. pjTO- 

 clastic rock ; there are at least two marked varieties among the fragments : — 

 one of scoriaceous aspect, with base rather dark from opacite,|and distinct felspar 

 crystals having glass enclosures, mostly plagioclase ; the other generally with 

 microlithic crystals, and some indications of flow-structure. It is very probable 

 that, as you suggest, the rock is an andesitic ash. It is rather decomposed iii 

 parts, secondary products being formed, and feri-ite replacing opacite. — T. G. B.] 



t [Rock from side of Clochnant. A crystalline rock, consisting of well-pre- 

 served augite, much decomposed felspar, and a eerpentinous mineral or minerals, 

 with iron peroxide, probably (in part at least) ilmenite. The felspar appears to 

 have been plagioclase ; a little of a monoclinic zeolitic mineral in radiating 

 aggregates is now present, possibly heulandite. The serpentinous mineral may 

 replace olivine ; but in some cases the form of the grains does not very well 

 accord with this supposition. There is one ci*ystal of a brown hornblende, a 

 variety which I have occasionally met with in old crystalline augitic rocks. 

 This is one of those coarse doleritic rocks common in Wales, and more likely 

 to have been intrusive than interbedded. — T. G. B.] 



J In a former paper (Q. J. G. S. vol. xxxiv. p. 872) I stated that Echitio- 

 spharitcs Davisii, E. halthicus, E. mammomis, Holojjella, and Holnpca were 

 Coniston-Limestone fossils ; the beds containing them have since proved to b« 

 in the Ashgill-Shalc series. 



