﻿Yol. 66.'] TREMADOC SLATES OE SOUTH-EAST CARNARVONSHIRE. 171 



about 100 feet at Tyddyn-dicwm to perhaps 300 feet in the Ynys 

 near Tremadoc. Within this crush-belt the slates are so flaked and 

 shattered by the jointing, that both cleavage and bedding are difficult 

 to observe. Soft, silky, or earthy in their texture, they are dark 

 enough in colour to have persuaded some prospectors to dig for coal 

 among them, and it was in the search for coal that the pisolitic 

 iron-ore was discovered. 



The pisolitic iron-ore masses, like all other rock-units within the 

 crush, occur in the form of augen or lenticular lumps which vary 

 from the size of a bean to that of a 100-ton schooner, and have a 

 distribution which seems to be sporadic all through the dark shale 

 of the crush-band. Until 1860 the ore was worked with great zeal; 

 but, as in the deeper workings the proportion of sulphur increased, it 



Pig. 3. — Sketch-plan of the surface from which the iron-ore has been stripped. 



Contoured plan 

 of the floor of Tremadoc Iron-Mine -*>'' 



The continuous lines indicate contours which 

 are drawn at intervals of 10 feet. 

 The broken lines are the cusps of the dip- 

 folds which divide the iron ore into its 

 augens. 



Scale: 1 inch = 50 fcet- 



became unprofitable. The plotting of the contour strike-lines of the 

 surface from which ore has been stripped in the Tremadoc mine 

 between Tremadoc and Glanmorfa gives the accompanying diagram 

 (fig. 3), which shows well the augen character of the large ore-units. 

 The puckering of the dip-surfaces between which the augen lie, illus- 

 trated in the second diagram (fig. 4, p. 172), is also characteristic. 



Though, from their mode of occurrence, the ore-masses must 

 be related to the crushing movements, the individual ore-masses, 

 whether large or small, show no signs of crushing, and the pisolitic 

 grains within them maintain their sphericity, right up to the surface. 

 The slates around may be rolled out to a regular sericitic mylonite, 

 and pieces of shale within the ore be of similar character, but the ore 

 itself is never in the very least affected. This, and the circumstance 



