GEOLOGY 109 



theory there would be a tendency for radioactive mineral matter to become concen- 

 trated in or near the superficial part of the earth, thus offering an explanation of 

 the results obtained by the Hon. J. R. Strutt in his well-known researches on this 

 subject. 



A theory of the evolution of the earth resembling in many ways that of Prof. Cham- 

 berlin seems to have been independently developed by Prof. E. H. L. Schwarz, of 

 Grahamstown, and is set forth in his work on Causal Geology (London, igio). In this 

 work he discusses some of the fundamental problems of geology in the light of his experi- 

 ence gained during extensive field-work in South Africa. He concludes that the earth 

 is throughout solid, rigid and cold. Local heat for volcanic phenomena is obtained by 

 pressure and friction along lines of movement in the earth's crust, and from radioactive 

 rocks. Such heat acting on water in the pores of the rocks gives rise to steam, producing 

 explosive outbursts, whilst if the rocks are readily fusible liquid lava may be formed. 

 In explaining the formation of that type of volcano illustrated by the caldera, or pit- 

 crater, of Kilauea, Prof. Schwarz assumes lateral movement of the rocks by simultaneous 

 thrust along two crossed horizontal faults, with depression of the enclosed area. The 

 origin of the depression is therefore similar to that of the fold-basins in many parts of 

 Cape Colony. 



As South Africa seems to have stood above sea longer perhaps than any other part of 

 the world, it has suffered superficial denudation to an exceptional extent, and Prof. 

 Schwarz believes that the crust is there so thin that the centrosphere has actually been 

 reached by the diamond-pipes. These channels are now occupied by materials which 

 may be regarded as samples of the highly basic rocks of the deep-seated interior, akin to 

 meteorites. 



Pacific and Atlantic Types of Rocks. In a presidential address to the Geological 

 Section of the British Association at the Portsmouth meeting of 1911, Dr. A. Harker, 

 of Cambridge, developed his views on the geographical aspect of petrology, dwelling 

 especially on the relation that may be traced between certain types of rock and the 

 tectonic features of the regions in which they occur. So far as the younger igneous rocks 

 are concerned, a curious relationship seems to govern their distribution over extensive 

 geographical areas. 



It was pointed out many years ago by Prof. E. Suess that the coastlines of the Pacific 

 and the Atlantic basins were characterised by two distinct types of structure. The 

 Pacific is bordered for the most part by mountain chains that are folded towards the 

 ocean, so that the outer side of the fold looks seawards. On the Atlantic coast, however, 

 such a structure is almost absent, and if folds exist the inner side generally faces the 

 ocean. After this distinction had been recognised Dr. Harker called attention to the 

 fact that the Pacific structure was characterised by igneous rocks of what is called the 

 calcic type, and the Atlantic structure by rocks of alkaline facies. The petrological 

 distinction indicated by these chemical expressions is now believed to be of fundamental 

 or genetic significance. All igneous rocks admit of classification in two great branches, 

 one of which is characteristically calcic or alkali-calcic in composition, while the other 

 being rich in potash and soda is known as the alkali branch. 



There is generally a regional separation between the two groups, the typical calcic 

 and the typical alkali rocks being characteristic of distinct petrographical regions, each 

 including subordinate provinces. The alkali group seems to be associated with areas of 

 subsidence believed to be due to radial contraction, whilst the calcic group is connected 

 with areas of folding, resulting from lateral compression. Viewed broadly, the calcic 

 branch predominates over the alkali branch, but many rocks of the latter type may be 

 hidden by submergence in sunken areas. 



According to Dr. Harker, the variations in igneous rocks have been brought about 

 mainly by differentiation of the original magma rather than by assimilation, or absorp- 

 tion of local rocks by the molten matter. But the relation between differentiation and 

 assimilation is a subject on which petrographical opinion is still much divided. 



(F. W. RUDLER.) 



