STRUCTURE OF EXISTING MOUNTAINS 391 



It is implied also by the geographical unity of the whole side of eastern 

 Australia during Cenozoic time. 



The 'New Zealand mountains tell a similar but more dramatic story 

 than the more stable land blocks of Australia. 



Structure of past Mountains 



The appearance of the surfaces of past mountains can be inferred only 

 from a comparison of their exposed cores or stumps, on the one hand, 

 and from the general form of existing mountains, on the other. 



It is natural in this connection to understand that there is little or no 

 knowledge to be had of ancient mountains formed of rocks with negligible 

 dip. Districts of ancient folding only are considered in this connection. 



Two things, however, are suggested at an early stage in this discussion : 



(a) The Eocene and Miocene mountains, of which the present mag- 

 nificent ranges of the earth appear to be revivals by vertical uplift 

 following on long cycles of denudation, exhibit wonderful folding phe- 

 nomena and evidence of igneous intrusives of plutonic type at the cores. 

 This is observable within the Tethyan area and at the locus of the inter- 

 ference of the Tethyan and Pacific controls. 



(b) The very old mountain systems show a remarkable development 

 of close folding. 



The areas occupied by mountains of Archeozoic and early Proterozoic 

 time were enormous, both in length and in width. This statement ap- 

 pears to hold, apparently, whether the areas considered be Eurasia, 

 America, Australia, Africa, or peninsular India. 



This fact alone, at first sight, would suggest that the influence of fold- 

 ing in ancient time had extended over immense areas. The rock outcrops 

 themselves of these old mountain cores also suggest an arrangement in 

 subparallel or confocal rings somewhat as the ranges of today, due allow- 

 ance being made for local variations during successive periods, such as 

 the slow wanderings due to land-wave interference, or cusps, which has 

 caused great deviations of strike, amounting to many hundreds of miles 

 in length in places. This is in marked contrast with the distribution of 

 post-Huronian mountains. 



^ . On and within the ancient nuclei of the continents the post-Huronian 

 sediments appear to be but little folded and altered. On the other hand, 

 the Paleozoic sediments are folded strongly in strips which are relatively 

 narrow on the outer belts, between the Archeozoic rings, but which pass 

 thence with gentle or negligible dip toward the continental nuclei. These 

 Paleozoic fold areas, however, are relatively wide as compared with the 

 rings of Cenozoic folding. 



XXVI — Bull. Geol. Soc. Am.. Vol. 34, 1022. 



