560 Dr. H. Hicks — Effect produced by Earth-movements. 



if intruded into the Pebidian beds, whilst in reality it is everywhere 

 bounded by faults, some of them of pre-Cambrian, others of post- 

 Cambrian age. Similar effects to those witnessed at St. David's and 

 in other areas in N. Pembrokeshire, may be seen in sections in 

 Carnarvonshire, especially in the area between the Menai Straits 

 and the Snowdon district. In the part of this area lying between 

 Carnarvon and Llyn Padarn, not only are the Cambrian rocks, by 

 reversed faults, made to appear to underlie pre-Cambrian rocks, but 

 even Arenig beds are faulted so as to dip under both. The move- 

 ments in this area have taken place mainly from S.E. to N.W. and 

 the rocks of the Snowdonian range have been driven westward by 

 repeated thrusts. 



In a section in Shropshire, extending from the Longmynd across 

 Caer Caradoc, Lower Palaeozoic rocks are faulted on the W. side 

 so as to appear to underlie the pre-Cambrian rocks of Caer Caradoc, 

 whilst on the E. as the result of thrust-movements, the older beds 

 have been hidden by much newer ones. Here the post-OrdoA'ician 

 movements were mainly, as in Carnarvonshire, from S.E. to N.W. 



The changes which have been produced in the rocks themselves 

 are also very marked. The granitoid rocks give evidence of having 

 been greatly crushed by the Earth-movements in pre-Cambrian 

 times, and in the lines of fracture secondary minerals have been 

 freely deposited. That these secondary minerals date back to pre- 

 Cambrian times is shown by the fact that the pebbles of these 

 granitoid rocks in the Cambrian conglomerates contain all the 

 evidences of the early crush with secondary minerals in the crush- 

 lines, in addition to those of subsequent fractures and deformation 

 by pressure after they had been entombed in the conglomerates. 

 In some places the granitoid rocks and diorites have suffered so much 

 from pressure that they have put on a distinctly gneissic aspect. 

 Some of the felstones in pre-Cambrian times were crushed so that 

 they were formed into felsitic schists, and fragments of these schists 

 occur frequently in the Cambrian conglomerates. Various dykes 

 in the pre-Cambrian rocks exhibit indications of having suffered 

 greatly from mechanical pressure in pre-Cambrian times, the diabase 

 dykes in the Dimetian being frequently cleaved so as to look almost 

 like slates. Fragments of these and of many other cleaved and 

 altered rocks are often found in the Cambrian conglomerates. 



In the Cambrian and Ordovician rocks the evidences of pressure 

 during subsequent Earth-movements are also abundant, and secondary 

 minerals have been freely developed along planes of cleavage and 

 in lines of fracture. The effects on some of these rocks near 

 thrust-planes are well exemplified by the remarkably distorted 

 condition of some of the fossils. In one case near St. David's an 

 Orthis, which in its normal condition was about seven lines in 

 width, was so distorted that it measured over 27 lines, and others 

 were still further drawn out so as to be almost unrecognizable. 



