Henry Kicks — On the Northern Pahwzoic Rocks. 249 



mentioned generic types, I have added the annexed diagrammatic 

 sketches, from which their leading peculiarities will be readily 



perceived. 



EXPLANATION^ OF PLATE IX. 

 Fig. 1. — Temnograptus {DicJiograptus) multiplex, Nich. Skiddaw Slates. Nat. size. 

 „ 2. — Trichograptus {Bichograptus) fragilis,W\Qh.. Skiddaw Slates. Nat. size. 



Fig. 2a. Portion of the same enlarged, 

 j^ 3. — Schizograptus {Bichograptus) reticulatus, Nich. Skiddaw Slates. Nat. size. 

 ,, 4. — Ctenograptus [Bichograptus] amtulatus, Nich. Skiddaw Slates. Nat. 

 size. Fio'. 4«. Portion of a branch of the same enlarged. 



III. — Some Considerations on the Probable Conditions under 

 WHICH THE Palaeozoic Kocks were Deposited over the 

 Northern Hemisphere. 



By Henry Hicks, F.G.S. 

 {Continued from page 218.) 

 Igneous Action and MetamorpMsm. — I have already referred to the 

 fact that igneous eruptions were frequent during Palasozoic time. 

 As far as there is any evidence at present, they do not appear to 

 have taken place during the Cambrian Epoch either in Europe or in 

 North America. In Europe contemporaneous tuffs or ashes are 

 found for the first time in the succession in the Arenig rocks ; and 

 in America about the same period, or perhaps a little earlier. By 

 referring to the Table, p. 156, it will be seen that the contempo- 

 raneous traps also occur only in the areas which had undergone 

 most depression, or where the greatest amount of sediment had 

 accumulated. Sir J. Herschel has urged as a reason for this that 

 the crust after a continued period of depression would descend to a 

 point where the heat would be sufficiently great to melt it through, 

 or to weaken it ; and that as a consequence fractures would take 

 place, and there would then be a corresponding rising of heat and 

 fiery matter from below, to that of the accumulation of the sediments. 

 That a great sea of molten matter occurred at that time at a depth 

 of probably no less than 25 miles, and not more than 50 miles, there 

 can, I think, be no doubt, for in no other way can we account for 

 the gradual depression of so large an area of the Northern Hemi- 

 sphere, and of the subsequent movements which took place. That 

 the melting point even then, however, could not have been much 

 nearer the surface than 25 miles, is evident from the fact that sedi- 

 ments which were buried at a depth of at least 50,000 feet were 

 scarcely at all altered by the combined heat and pressure to which 

 they were subjected. 



In the south-west part of Wales the succession from the base of 

 the Cambrian to the Carboniferous is, as already stated, perfectly 

 continuous, and there is no indication anywhere of a break. The 

 same folds have affected the whole series, and they lie in perfect 

 conform ability the one on the other. All the beds also, from the 

 base upwards, where there are no intrusive masses, are free from 

 any of the usual signs of metamorphism, and the fossils are there- 

 fore well preserved. The reason why these beds are so little altered 

 may be due to the fact that this area was intermediate in position 



