16 FORMATION OF THE EARTH 



The oldest gneiss and granite is always found in the con- 

 cavities of stratified layers folded by terrific lateral compression, 

 and these folds are usually alternately concave and convex, 

 constituting what geologists call synclines and anticlines. 



The anticlines are naturally highly elevated, and correspond 

 to the summits of the mountain chains formed by this 

 crumpling. These chains were not formed by a single action. 

 The solid crust of the earth, being compelled to follow the 

 contour of the molten sphere, which, owing to the gradual 

 cooling of the globe, contracted more swiftly, became folded 

 in such a way as to preserve its surface intact while at the same 

 time it shrank and diminished in volume. Contrary, however, 

 to geometrical conjecture, from that epoch at which the earth 

 becomes accessible to our observation, the continents did 

 not form prominences directed towards the meridians, as the 

 tetrahedral theory would have us believe, but rather rings or 

 bands oriented parallel with the equator. This is either 

 because the centrifugal force resulting from the rotation of the 

 earth has contributed to their formation or because the cooling, 

 always more intense at the poles, has caused the formation of 

 powerful barriers that could resist a thrust in the direction of 

 the poles, tangentially to the meridians. The first of these 

 bands was formed near the North Pole ; we do not know 

 whether there was another corresponding to it at the South 

 Pole, the southern hemisphere being to-day largely concealed 

 beneath the ocean. It was in the course of its formation that 

 the Circum-polar gneisses were folded ; the direction of these 

 folds indicates the position of the oldest of the mountain 

 chains, the Hnronian, so-called because the traces it has left 

 of its existence are particularly visible in the neighbourhood of 

 Lake Huron in the American continent ; but it once extended 

 from thence to Greenland, northern Scandinavia, and Siberia. 

 Later it was surrounded by a second chain, situated more to 

 the south, called the Caledonian because it is definitely recogniz- 

 able in the Grampian Hills of Scotland ; it extends into 

 Scandinavia, and appears again in the Green mountains of 

 Vermont, in the State of Maine and in the Appalachians. 

 Still later, and always further to the south, rose the Hercynian 

 chains, whose name recalls the vast Hercynian forest, which in 

 the time of Caesar covered the mountains of the Black Forest, 

 the Harz, the Erzgebirge, and the Riesengebirge, and extended 



