192 Proceedings of the Royal Physical Society. 



period, and our knowledge of the marine fauna is not suffi- 

 cient to enable us to deny the possibility of moderate 

 fluctuations in the temperature of the seas of early Palaeozoic 

 times. Moreover, we must not forget that there were then no 

 such barriers to migration as now exist. If the conditions 

 became temporarily unsuitable, marine organisms were free to 

 migrate into more senial waters and to return to their former 

 habitats when the unfavourable conditions had passed away. 

 The uniform climate so characteristic of the Cambro-Silurian 

 period appears to have prevailed likewise during the later 

 stages of the Palaeozoic era. This we gather from a general 

 consideration of the floras and faunas and their geograjjhical 

 distribution. The dry land, as we have seen, continued to 

 increase in extent ; but vast areas of the primeval con- 

 tinental plateau of the globe still continued under water, 

 and currents from southern latitudes flowed unrestricted 

 into Polar regions. During the protracted lapse of time 

 required for the formation of the later Palaeozoic systems, 

 several periods of high eccentricity must have occurred. 

 But, so far as one can judge, the disposition of the larger 

 land-areas was never such as to induce a true Ice Age. 

 Nevertheless, we are not without evidence of ice-action in 

 Old Ked Sandstone, Carboniferous, and Permian strata. 

 And it seems to me probable that the erratic accumulations 

 referred to may really indicate local glacial action, of more 

 or less intensity, brought about by such lowering of the 

 temperature as would supervene during a period of high 

 eccentricity. It is true we may explain the phenomena by 

 inferring the existence of mountains of sufficient elevation — 

 and this, indeed, is the usual explanation. But it is 

 doubtful whether those who adopt that view have fully 

 considered what it involves. Take, for example, the case of 

 the breccias and conglomerates of the Lammermuir Hills, 

 which have all the appearance of being glacial and fluvio- 

 glacial detritus. These deposits overlie the highly denuded 

 Silurian greywackes of Haddingtonshire in the north and 

 of Berwickshire in the south, and have evidently been 

 derived from the intervening high grounds — the width of 

 which between the Old Pted Sandstone accumulations in 



