452 E. T. Newton — Lemmings, etc. in the Thames Valley. 



movement of the sea-floor, allowing even of local denudation ; while 

 hollows may have been formed by the explosive action, though there 

 is a marked absence of the tuffs with slate-fragments that are so 

 conspicuous on the slopes of Cader Idris. 



The coarse hornblende-ash, with its handsome crystals, a rock 

 perhaps unique in Wales, passes into more fine-grained types as the 

 summit of the hill is neared. At the close of activity we find, both 

 on Ehobell Fawr and Moel Cors-y-garnedd, a return to normal 

 sedimentation, tbe formation of shaly layers, and the incoming of 

 the beds of grit. The latter were deposited by waters flowing not 

 only over ancient quartzites, but over andesitic cones, which had at 

 one time risen well above the sea. 



To the east, in the area of Allt Lwyd and Ddualt, ordinary sedi- 

 ments were contemporaneously formed ; and when the volcanic 

 activity diminished, and the vast mass of erupted matter sank into 

 the shattered and yielding Lingula Flags thi'ough which it had been 

 projected, a common undisturbed sea-floor was re-established, and 

 the sandy layers were deposited over all alike. Tbe intrusive 

 dolerites, which, with the other rocks, we hope to subject to closer 

 petrological examination, may or may not prove to be connected 

 with the fragmental deposits of Ehobell Fawr. In any case we 

 cannot but regard this volcano as commencing in Tremadoc times, 

 and as a precursor of the great Ordovician eruptions ; and the 

 existence of such sporadic outbursts must be taken into account as 

 a disturbing cause when we seek to correlate the strata below the 

 Middle or basement Arenig of North Wales with those of other areas. 



We append a section, in the preparation of which we have been 

 greatly aided by the new Ordnance maps on the scale of six inches 

 to the mile. Considering the absence of the hornblendic tuffs on 

 the east of Moel Cors-y-garnedd, and the close resemblance of the 

 beds beneath the grit to those on the summit of Ehobell Fawr, we 

 have represented the great mass of the tuffs at the former point 

 as cut out by an oblique fault, which would account for the low 

 position of the grit-series and its proximity to the black Lingula 

 Flags. The vertical scale of the section is the same as the horizontal, 

 so that distortion of dip or of the relations of the several exposures 

 of the grit is, we trust, avoided. 



During our examination of the area of Ehobell Fawr, we have 

 been frequently in communication with Mr. G. J. Williams, F.G.S., 

 who has remarked, among other notes, upon the similarity of a 

 specimen of the grit of Moel Cors-y-garnedd and the typical Garth 

 grit of the Ffestiniog area. 



V. — On the Occurrence of Lemmings and other Eodents in the 



Brick-Earth op the Thames Valley. 



By E. T. Newton, F.G.S., F.Z.S. 



"TVTOT WITHSTANDING the large number of Mammalian remains 

 _1_M which have been found in the Brick-Earth of the Thames 

 Valley, they have for the most part been portions of the larger 

 forms, and very few examples of the smaller Mammals have hitherto 



