Reports and Proceedings — Geological Society of London. 187 



emergence after passing on the farther side of the centre of the 

 earth ; the increase of length of wave-path means a longer interval 

 between origin and emergence, and the sudden increase of interval 

 at about 130° is explicable in this way. An alternative explanation 

 is that the second-phase waves are extinguished at these distances, 

 but this explanation is not regarded as probable by the author. 



Either alternative leads to the conclusion that, after the outermost 

 crust of the earth is passed, there is no indication ot any material 

 or rapid change of physical condition, nor probably of chemical 

 composition, until a depth of about six-tenths of the radius is reached; 

 but that below this there is a rapid passage to matter which has 

 very different physical properties, if not also diifering in chemical 

 constitution. Without advancing any hypothesis as to the nature of 

 this difference, the author points out that it will have to be x"eckoned 

 with in any theory of the earth. 



2. " The Tarannon Series of Tarannon." By Miss Ethel M. R. 

 Wood, B.Sc. (Communicated by Professor Charles Lapworth, M.Sc, 

 LL.D., F.R.S., F.G.S.) 



A general historical review is given of the existing state of 

 knowledge respecting the character and fossils of the strata included 

 under the general title of ' Tarannon ' on the maps of the Geological 

 Survey in Wales. The Tarannon strata are most fully developed in 

 the Llanbrynmair-Tarannon district, after which they are named; 

 and the present paper gives the results of a detailed survey of the 

 entire Tarannon Series as there exhibited. Various sections — typical 

 and confirmatory — are described, showing the local sequence of the 

 strata. Lists of the contained graptolites are given, and the species 

 are paralleled with those from the corresponding beds of the South 

 of Scotland, the Lake District, North Wales, Central Wales, and 

 Sweden, demonstrating the similarity of the graptolitic succession 

 in all these districts. 



The authoress establishes the following local sequence : — 

 C. Wexlock. Series. 



Gb. Fynyddog Group. 

 Ca. Nant-ysgollon Group. 



Zones of Monograptus riccartonensis aud Cyrtograptus Murehisoni. 

 B. Tarannon Series. 



'Bd. Dolgau Group. (Zone of M. crenulatus.) 

 Be. Talerddig- Group. (Zone of M. griestonensis.) 

 BJ. Gelli Group. (Zone of M. crispiis.) 

 Ba. Brynuiair Group. (Zone of M. turriculatus.) 

 A. Llandovery Series. 

 Ac. Twymyn Group. 



Zone of M. Sedgmckii. 

 Zone of Cephalograptus cometa. 

 AJ. Dolgadfan Group. 



Zone of M. convolutus. 

 Zone of M. fimhriatus. 

 Aa. Faclidre Group. 



Zone of Dimorp/iog)-aptHs Swanstoni. 



The Tarannon Series in this district has a maximum thickness of 

 3,500 feet, but thins somewhat as it is traced north-westward. It 



