2.50 Henry Hicks — On the Northern Falceozoic Eochs. 



between the more disturbed and perhaps deeper area of North 

 Wales, and the sloping pre-Cambrian ridge to the south.^ It 

 was therefore most favourably placed, and was almost out of the 

 influence of the molten matter which elsewhere was being so 

 generally injected among the sediments, and which produced 

 metamorphism everywhere in its neighbourhood. In North Wales 

 all the series show indications, more or less, of being altered by 

 heat, and as volcanos were then active in that region, it is evident 

 that the change is chiefly to be attributed to proximity to these 

 intrusions. The portions of the old pre-Cambrian crust brought up 

 in Pembrokeshire at the base of these unaltered beds are highly 

 metamor23hosed, and they were doubtless in this state before they 

 were depressed to receive the Cambrian and succeeding sediments, 

 for every part of this old pre-Cambrian land that has been exposed 

 is found more or less in that condition. As all the Cambrian 

 and Silurian rocks, however, are now consolidated, it is of course 

 important to know what degree of heat is necessary to produce 

 that state only, and what to produce the further change of meta- 

 morphism. Fortunately, by watching the effects produced by a 

 trap-dyke in passing through any series, it is possible to arrive 

 at some idea of the amount. 



In my paper " On the Occurrence of Phosphates in the Cambrian 

 Eocks," 2 the change effected by heat derived from a trap-dyke on a 

 sedimentary rock (in the Menevian group) and in its organic contents 

 was shown. The chief alteration produced by the intrusive mass 

 in this rock for a considerable distance on each side was to cause 

 cleavage and a deepening of the colour, to remove carbonate and 

 phosphate of lime and all evidences of such substances as the horny 

 texture of the shell of Brachiopoda, Crustacea, etc. 



Now, as these Menevian rocks in Pembrokeshire must have been 

 at a depth of at least 40,000 feet before the movements took place 

 which brought them to the surface at the close of the Palseozoic, it is 

 evident, from the state of the rock at present, and of the organic 

 remains, that the heat at that depth was not sufficient to produce 

 much change in the rock or in the fossils — the shells of the fossils 

 being quite perfect, and apparently unaltered by heat, and phosphate 

 and carbonate of lime present in abundance. This fact, in my 

 opinion, is of more importance, in regard to the seat and depth 

 of internal heat, than any evidence that can be derived from exami- 

 nation of the temperature in mines and deep sinkings at present. 

 For the depth was much greater, and the evidence is derived from 

 an earlier period in the history of the globe. To my mind this 

 evidence tends strongly to prove that the heat has a deep-seated 

 source, and that it is not dependent on actions going on in the outer 

 crust only. Prof. Mohr's recent experiments ^ to show that the rate 

 of increase of temperature diminished in a constant ratio with the 



^ A portion of the Malvern range is probably in the line of, and formed part of 

 this ridge. 



2 Quart. Journal Geo). Soc, August, 1875, 



3 Neues Jahrbuch, 1875, 4tli part, p. 371. 



