﻿that the largest ore- 

 bodies occur near the 

 top of the crush - 

 zone, where it abuts 

 upon the metamor- 

 phic aureoles x of the 

 late intrusive dole- 

 rites, has led me to 

 urge that the ore is 

 of secondary or m eta- 

 somatic origin, and 

 owes its distribution 

 to the position of the 

 fault. 



The country -rock 

 about the mines con- 

 sists always of blue- 

 black or dark leaden- 

 grey slates, which 

 rust to an uniform 

 dead brown colour. 

 Most of the slates 

 seem devoid of fossils, 

 but at Ynys - galch, 

 Pen-syflog, and the 

 old tramway-cutting 

 50 yards north-west 

 of the Tremadoc mine, 

 Climacograptus scha- 

 renbergi and Dicello- 

 graptus sextans occur 

 in some abundance. 

 It was from the debris 

 turned out of the iron 

 trial-holes at Tyddyn- 

 dicwm that Salter and 

 later observers have 

 obtained the grapto- 

 lites which they re- 

 cord. The country- 

 rock of the pisolitic 

 iron -ore is therefore 

 of Llandeilo age, and, 

 like the graptolite- 

 bearing rock in the 

 trench at Tyddyn- 

 dicwm, was originally 

 of that open -water 



1 Geol. 

 p. 422. 



Mag. 1907, 



