252 Henry Hicks — On the Northern Pakeozoie Rocks. 



ditions prevalent during the Carboniferous period of Paleozoic time. 

 In the cycle of changes which took place in pre-Cambrian times, we 

 find first great and high continents, with small marine areas in the 

 northern latitudes, and consequently a climate probably of extreme 

 cold. This would, of course, be most unfavourable to life, and if 

 life then existed on the globe, it would be in the areas probably about 

 the Equator, where heat and moisture would favour change and 

 progress. As the continents became depressed, and water spread 

 more freely over the Northern Hemisphere, as we know it did to- 

 wards the close of the Lower Laurentian, a mild climate would be 

 the consequence, and the marine and land life would in a degree be 

 like that towards the close of the Palaeozoic in the next cycle. At 

 the close of the Lower Laurentian, large areas were upheaved, and 

 though they were again partly submerged, other upheavals took 

 place, and the continents were then raised to the state in which they 

 were in before the depression commenced in which the Cambrian 

 rocks were deposited. The first cycle, therefore, in the Northern 

 Hemisphere was unfavourable as a whole to the development and 

 progress of life in the higher latitudes, the first and last parts being 

 periods probably of cold, and the middle only mild and moist, or 

 such as would favour life. 



The early Cambrian, for the same reason, was a period unfavour- 

 able to life in the higher latitudes ; and hence we find that forms 

 of life suitable to the changes which were gradually taking place 

 migrated towards the areas from southern points. That it was a 

 cold epoch is evident from the fact that the pre-Cambrian continents 

 occupied very extensive areas in the higher latitudes, and that they 

 were traversed by mountainous ranges attaining in some cases to 

 great heights. At no time since, unless in the Glacial period, does 

 there seem to have been so much land in the higher latitudes, and 

 it is, therefore, reasonable to suppose that in the earlier stages at 

 least of the epoch the climate was one of great cold. As the land 

 became submerged, a state of gradual transition to a milder climate 

 would be the consequence. 



Along with the increasing distribution of water in the Northern 

 latitudes we have indications in the faunas that a change was taking 

 place in the temperature of the seas over those areas. For, in addi- 

 tion to the forms (Trilobites, etc.) which were present at first, and 

 doubtless capable of inhabiting seas of low temperature, we find 

 others gradually coming in which could only have lived in temperate 

 or warm seas. The presence of an abundance of corals and certain 

 forms of Cephalopods, towards the close of the Lower Silurian, is 

 strong evidence that by this time, at least, warm currents or seas 

 of moderately high temperature prevailed over the areas to a tolerably 

 high latitude on both sides of the Atlantic. 



The warmer or more equatorial points seem to have been the 

 home of the earlier forms of life, and it was there probably that 

 the chief changes in development took j)lace. 



From these points they gradually migrated northwards as the 

 conditions became favourable, and the groups as they successively 



