﻿166 Geological Society : — 



reached their 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 sndden 

 increase of interval at about 130° is explicable in this way. An 

 alternative explanation is that the second-phase waves are extin- 

 guished 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 of 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 differing in chemical 

 constitution. Without advancing any hypothesis as to the nature of 

 this difference, the author points out that it will have to be reckoned 

 with in any theory of the earth. 



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

 Wood, D.Sc. 



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. Wenlock Series. 



Cb. Fynyddog Group. 

 Ca. Nant-ysgollon Group. 



Zones of Monograptus riccartonensis and Cyrtograptus 

 MurcMsoni. 



B. Tarannon Series. 



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

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

 Bb. Gelli Group. (Zone of M. crispus.) 

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



A. Llandovery Series. 

 Ac. Twymyn Group. 



Zone of M. ScdgwicMi. 



Zone of Cephalograpkcs cometa. 



