2 ( .) I) Geological Society. 



f Sooty-black mudstones with Peltura scarabaoides. 

 Blue-black inudstone with Agnostus trisectus. 

 Iolgei.ly I gj ac ]- plates, with calcareous bands often crowded 

 Series. witll q^^ l en ticularis. 



[ Dark flaggy slates with Parabolina spinulosa. 



I Grey-blue slates and flags crowded with Lingulella 

 davitii. 

 Grey flags and grauwacke with some coarser 

 bands (1800 feet thick). 



Maentvveog Rusty grey and blue slates with thin bands of 

 Series. f'elspathic grauwacke. 



The folding, the cleavage, the faulting, and the jointing of the 

 rocks are described, and an attempt is made to show some 

 relationship between the various stress-phenomena which have 

 produced these structures. 



The great fault through Penmorfa is interpreted as a thrust- 

 plane hading gently to the north-east. It is described as bounding 

 two districts which are of very different structural type, and is 

 supposed to form the lowest sole of the group of thrust- planes 

 which follow the southern margin of the Snowdonian mountain- 

 tract. 



The well-known pisolitic iron-ore of Tremadoc is shown to 

 follow the line of this fault, and is thought by the Author to be 

 of the nature of a metasomatic veinstone. 



Direct evidence of overthrusting has been got from a study of 

 the graptolite-bearing Llandeilo rocks of Tyddyn-dicwm, which 

 have been exposed in two artificial trenches dug for the purpose; 

 and the distribution of the andesitic volcanic series in lines of 

 detached lenticles among the Grej - Slates is described as evidence 

 of a similar reduplication of the newer rock-series of the north- 

 eastern district on a more extended scale. 



The actual lines of major thrust-planes, other than the Penmorfa 

 ] ; ault, have not been discovered ; but, from the broad-spreading 

 character of the sills of gabbroid dolerite, the Author infers that 

 these have come in along the thrust-planes. 



The petrographical characters of these quartziferous and 

 hypersthene-bearing dolerites are not dealt with, but it is noted that 

 the dolerites are (1) unaffected by cleavage and faulting; and 

 (2) have metamorphosed rocks which were already cleaved, cut, 

 and reduplicated by the thrust-faulting at the time of their 

 intrusion: in this the Author joins issue with previous observers 

 in regard to their age. 



The Glacial and post-Glacial accumulations are also described 

 in outline. 



