part 2] ITS STRUCTUKE AND EOCK-SUCCESSION. 157 



Thichness in feet. 



(1) Fossiliferous blue-grey mudstone, full of fossils 12 



(2) Pisolitic limestone in large concretionary masses measuring 



3x2 feet 3 



(3) Concretionary calcareous mudstone, almost a limestone in 



places, and with numerous scattered pisolitic grains ; blue- 

 grey when fresh, but weathering to a dirty cream-colour ... 10 



(4) Dark bluish mudstone, with a few scattered pisolitic grains, 



and but few fossils : 10 



[The total thickness of these rocks, as seen in the quarry, is 

 from 25 to 30 feet.] 



The same characteristic fossils occur throna-hout : — 



Orthis liirnantensis M'Coy. 

 Orthis sagittifera Davidson. 

 Dalmanella elegantula (Dalman). 



StropJwmena siluriana Davidson. 

 Platystrophia hiforata (Schlotheim). 

 Monticulipora fibrosa (M'Coy). 



The pisolitic limestone has been so well described by Fulcher 

 that no further description need be given, although the pisolitic 

 grains which he defines as ellipsoidal in form are only so in the 

 direction of the cleavage. 



On the opposite side of the Hirnant several small slate-trials 

 have been made ; beneath the lowest of these mudstones similar 

 to those last described are seen dipping into the hill at 75°, and 

 these pass upwards into a similar rock but containing slaty bands. 

 In all these the Hirnant fauna is to be found (fig. 8). The slate- 

 bands increase steadily in importance, and without any break the 

 Hirnant Beds pass over into a slate series to which I have given 

 the name Cwm-yr-Aethnen Beds: these appear to be about 

 350 feet thick. They are, however, capable of a twofold grouping, 

 both on lithological and on palaeontological grounds : the lower 

 group (150 feet) consists of hard, fine, blue-grey slates with some 

 banding, yielding Upper Birkhill ■ graptolites : — Monograptus 

 sedgwichi (Portlock), Climacograpttis scalar is (Hisinger), Ortlio- 

 graptus helluliis (Tornquist), Glyptograptiis serratus Elles & 

 Wood, and Petalograptus sp., a fairly typical assemblage of the 

 Zone of M. sedgwicM. These pass up into somewhat softer 

 slates, more definitely banded in lighter tones than the lower group, 

 and yielding well-preserved graptolites in the blacker bands : — 



Monograptus crispus (Barrande). | Monograptus discus Tornquist. 



Monograptus turricidatus (Barrande). , Moiiograptus becki Barrande. 



Monograptus priodon Brown. I Monograptus marri Perner. 



Monograptus nudus Lapworth. | 



V. Stetjcture of the Disteict. 



The structure of the district is somewhat complicated, although 

 when the main principles underlying the general plan are under- 

 stood it appears less complex than might be thought at first sight. 



The interpretation here given, which seems to accord with the 

 facts now observed, necessitates a modification of the views hitherto 



