part 2] ITS STEUCTURE AND E0CK-STJCCESSI02S". 143 



Rliivvlas Mudstone and Limestone. 



Following on the Gelli-grin Ashy Series come the Khiwlas 

 Beds, another mudstone series with impersistent liinestones at its 

 base. There is, however, a marked change in lithological character, 

 whether limestone be developed or not : the characteristic beds are 

 very fine pale-grey mudstones with some sandy mud stones, of so 

 even a texture that they might be termed pasty ; these are, as a 

 rule, highly cleaved, and sometimes contain definite calcareous 

 concretions. The coarser micaceous sandy beds which lie above 

 them are less cleaved, but are still pale gre^^ in colour. The base 

 of these Rhiwlas Beds appears to be calcareous west of a north- 

 and-south line running through Gelli-grin. There seems to be 

 only one locality in the district where the Grelli-grin and Rhiwlas 

 Beds are both calcareous, and this probably — in 'part, at any rate — 

 accounts for the confusion that has existed in the past with regard 

 to these beds, although they really differ conspicuously in fossil 

 contents as well as in lithological character. The Khiwlas Lime- 

 stone is a pale-grey, minutely-lenticular, fragmentary limestone, 

 and doubtless on this account is unsuitable for burning for lime : 

 hence, unlike the limestones at the lower horizons, it has never 

 been worked for that purpose. When weathered it presents much 

 the same external appearance as the lower limestones, being charac- 

 teristically honeycombed. It commonly occurs also in wall-like 

 stretches varjdng from 10 to 15 feet in thickness, but never, so far 

 as my knowledge goes, exceeding the latter thickness. 



The characteristic fauna of these Khiwlas Beds is the same, 

 whether the limestone be developed or not, and a noteworthy 

 feature is the almost total disappearance of all the large brachiopods 

 which form so essential a part of the fauna of the Gelli-grin Beds. 

 Certain small brachiopods occur ; but it is the trilobite fauna that 

 is the most conspicuous element, and the trilobites also are very 

 different from those found in the lower beds, the most typical 

 forms being Fhillipsinella parahola^ Stauroceplialus murchisoni, 

 Gheirurus himucronatus, LicJias laxatits, large Illsenids, and 

 Agnostus agnostiformis. There is a relatively greater number of 

 individuals in the limestone, but the fauna is the same in the 

 pasty mudstone ; it gradually dies out in the overlying beds, and 

 has completely disappeared by the time that the Moel-frj'-n Beds 

 have developed in force. 



Moel-fryu Sandstones. 



These sandstones, which are first seen merely as bands in the 

 Khiwlas Mudstones, become eventually massive, and occupy a 

 considerable area of ground on Moel-fryn and on the hills north of 

 the Hirnant valley beyond Aber Hirnant. They are a very mono- 

 tonous and uninteresting set of beds, somewhat micaceous and of 

 the same pale-grey colour throughout, Avithout a trace of an}'- 

 fossils, so far as I have been able to discover. 



