80 MR. H. LA.PWORTH 01^ THE [Feb. I9OO, 



in a state of excellent preservation — skim, so to speak, along the 

 surface for a short distance, then dive under ; and, if the slab be 

 sufficiently thin, reappear on the opposite face. Flaggy blue shales 

 are exposed in the group here and there, and it is in these that 

 the investigator should search for fossils. The cleavage has not 

 been of sufficient intensity to penetrate these harder beds. The 

 group is somewhat ferruginous, as may be gathered from the occur- 

 rence of thin seams of iron-pyrites, which takes the form of small 

 globules. The flags are generally pale grey or full grey, but in 

 the centre of the group they are of a dark bluish-black. In the 

 lower beds of the zone the chief fossils are Monograptus communis, 

 Lapw., M. Jimbriatus, Lapw., and Petalograptus palmeus var. 

 latus, Barr. 



Overlying these comes a thickness of about 20 feet of shales 

 with Monograptus triangidatus, Hark, and Diplograptus sinuatus, 

 Nich., in addition to the foregoing species. Monograptus triangu- 

 latus is exclusively confined to this band. The last 60 feet of the 

 zone yields : — 



Diplograptus tamariscus, Nich. 



magnus sp. nov. 



Petalograptus palmeus var. latus. 



minor, Elles. 



Climaeograptus undulatus, Kiirck. 



normalis. 



rectangular is. 



Orthoceras sp. 



Diplograptus magnus is limited to the topmost 25 feet, and makes 

 its first appearance in company with Climacograptus undulatus. 

 Monograptus leptotlteca also is first found in these topmost beds. 



The total thickness of this zone is about 150 feet. 



The whole of the foregoing division — the Ddol Shales, — 

 including the zones of Monograptus tenuis (Ac t ), Monograptus 

 cyphus (Ac 2 ), and Monograptus Jimbriatus (Ac 3 ), is named after 

 Ddol Farm (pron. ' Thole/ like ' those '), which is built on the 

 central beds of the division. 



(d) Gigrin Mudstones. 



Calcareous-Nodule Beds (Ad x ). — A short distance belowthe 

 masonry-outlet to the Wye overflow-pipe on the Birmingham aque- 

 duct the fourth and highest division of the Gwastaden Group begins. 



The change between this division and the underlying one is 

 remarkable for its suddenness. So great is it that a knife-blade 

 can be inserted into the plane at which one group ends and the 

 other begins. This would lead the observer to suppose that a 

 fault existed at this point. Indeed that may be the case, but 

 such a fault can have no great throw, as detailed mapping has 

 shown no alteration in the thickness of either of the groups to 

 the eastward or westward. 



The Gigrin Mudstones fall into three distinct lithological 

 divisions. The lowest of these may be summarized as a series of 



Monograptus communis. 



jimbriatus. 



attciuatus, Hopk. 



Icptotkcca, Lapw. 



gregarius, Lapw. 



argutus, Lapw. 



crenularis, Lapw. 



Diplograptus sinuatus. 



