Reports & Proceedings — Geologists' Association. 95 



Arenig rocks are a series of arenaceous strata (the Aber^astle and 

 Porth Gain Beds) which, correspond to the ' Nesurefus beds ' of 

 Ramsey Island. These strata are in faulted relationship to the 

 Cambrian, so that the true base of the Arenig is not seen. The 

 arenaceous beds pass upwards without break into Tetragraptus shales, 

 which are in turn succeeded by the bifidus beds. Llanvirnian 

 volcanic rocks are represented in one part of the district by the 

 Llanrhian Volcanic Series, which begins high up in the zone of 

 Didymograptus bifidus, and in another part by the Hurchisoni Ash, 

 which forms the base of the D. Murchisoni zone. The Llandeilo 

 Series compares closely with that of Carmarthenshire, and does not 

 contain any volcanic rocks as was at one time supposed. 



Contemporaneous igneous rocks occur at two horizons : 



(1) keratophyres at a high horizon in the Tetragraptus shales, and 



(2) quartz- keratophyres (soda-rhyolites) towards the top of the 

 D. bifidus beds. The intrusive rocks (diabases) belong to two types, 

 (1) sub-ophitic quartz-diabases, and (2) ophitic diabases without 

 quartz. Both types were intruded earlier than the main folding, and 

 consequently earlier than the cleavage and faulting. 



A great north-westerly line of disturbance — the Pwll Strodyr 

 Fault — cuts across all other structures, and brings on entirely different 

 groups of strata. 



II. — Geologists' Association. 



January 8, 1915.— George W. Young, F.G.S., P.Z.S., President, 



in the Chair. 



The following lecture was delivered: " The Value of Graptolites 

 to the Stratigraphical Geologist." By Miss Gertrude L. Elles, D.Sc. 



The value of Graptolites as chronological indices depends upon the 

 rapidity of their evolution and their wide distribution in space. The 

 stages in evolution correspond to a great extent with the strati- 

 graphical classiti cation in common use, though at certain horizons 

 there are characteristic ' bursts '. Detailed knowledge of the different 

 species is in no way necessary for the recognition of the different 

 horizons. The nature and value of a Graptolitic zone depends on the 

 assemblage of characteristic forms. 



III. — Edinburgh Geological Society. 



December 16, 1914. — Dr. R. Campbell, President, in the Chair. 



The following papers were read: — 



1. " The Whinstone Dykes of the Great Cumbrae." By Thomas 

 Bond Sprague, LL.D. 



Dr. Sprague described the whinstone dykes of the island, and the 

 raised beaches, which form prominent features of the topography of 

 Cumbrae. 



