﻿170 ME. W. G. PEAKE SIDES ON THE [May ICJIO, 



Tremadoc trilobites were found in the rocks at the foot of the 

 eastern trench, and graptolites freqnent in some of the rocks of 

 the eastern were abundant at the top of the western trench. In 

 the making of these trenches it was found that the Tremadoc 

 rocks of the sole of the thrust are little affected by induced minor 

 cracking, but the Ordovician slates are cut again and again by 

 small overthrust-faults, each a little steeper than the great plane 

 of discontinuity at their base. 



In the eastern trench the Ordovician rocks are often gnarled and 

 shivered beyond description. They break into sheared and slicken- 

 sided, augen or spearhead-shaped masses, which are set with their 

 long axes a little more nearly north and south than the dip of 

 the thrust, which is here striking N. 60° W. and dipping at 25° to 

 N. 30° E. 



In the western trench the Llandeilo rocks are partly pyritized 

 and somewhat mineralized, and so, although broken by the minor 

 thrust-planes, form larger augen within which the slates retain 

 their bedding. Prom these augen the graptolite fauna recorded in 

 the Rep. Brit. Assoc. (Belfast) 1902, p. 599, was obtained. 1 In 

 making the trench it was noticed that there are three kinds of 

 graptolite-bearing rock characterized respectively by: — 



(1) Swarms of Climacograptus schare?ibergi, 



(2) Nemagraptus gracilis, 



(3) Climacograptus peltifer, 



each distinct one from the other. The occurrence of these in the 

 trench is plotted in the diagram, which surely gives abundant proof of 

 reduplication by faulting. Though the graptolitic assemblages in the 

 three bands are quite distinct, the evidence before us is not sufficient, 

 to make it clear that the bands belong to distinct graptolite zones. 



The Pisolitic Iron-Ores and Tyddyn-dicwm Shales. 



The rocks next above the Penmorfa thrust-plane are gnarled,, 

 shivered, and kneaded into a regular augen-schist through a thick- 

 ness (measured at right angles to dip and strike) which varies from 



1 The following list, kindly prepared for me by Miss G. L. Ellcs, D.Sc., 

 includes all the specimensof graptolites from Tiddyn-dicwm which are preserved 

 in the Sedgwick Museum to January 1st, 1910 : — 



Didymograptus superstes, Lapw. 

 Dicellograptus sextans, Hall. 

 Dicellograptus intortus, Lapw. 

 Dicellograptus divaricatus, Hall. 

 Dicellograptus moffatensis, Carr. 

 Dicranograptus ramosus, Hall. 

 Dicranograptus rectus, Hopk. 

 Dicranograptus ziczac, Lapw. 

 Dicranograptus fur catus, var. minimus, 



Lapw. 

 Dicranograptus nicholsoni, Hopk. 

 Nemagraptus gracilis, Hall. 

 Climacograptus schdrenbergi, Lapw. 



Climacograptus bicornis, Hall. 

 Climacograptus bicornis, var. peltifer, 



Lapw. 

 Climacograptus antiquus, Lapw. 

 Climacograptus antiquus, var. bursifer 



(Elles & Wood). 

 Amplexograptus perexcavatus, Lapw. 

 Glyptograptus teretiusculus, His. 

 Orthograptus priscus, E. & W. 

 Orthograptus whitfieldi, Hall. 

 Glossograptus hincksii, var. fimbriatus, 



Nich. 

 Cryptograptus tricornis, Carr. 



