DISTRIBUTION AND ARRANGEMENT OF PAST SYSTEMS 387 



For Australia the outer arcs of schists pass through ^ew Guinea and 

 the New Zealand or South Pacific festoon. 



It is believed by some observers that these outermost belts of schists 

 represent Archeozoic types, but it is probable that the majority of them 

 may be referred hereafter to periods very much later than the Archeozoic. 

 It is to the great nuclei and to the second ring that we may look with 

 confidence to the development of the Archeozoic folds. These nuclei and 

 rings of schist are all separated by reciprocal valleys and plains or by 

 land and sea troughs. 



Of the most ancient it may be said, possibly, that they passed outward 

 from the nuclei in a series of land crests and troughs, the eroded cores of 

 which possess mainly a general tilt or inclination toward Tethys and the 

 Pacific. The evidence of the influence of the Atlantic, Southern, and 

 Indian oceans should be remembered also in this connection. Eeference 

 is made in another place to this extra-Pacific influence. 



This growth by outward extension of rings and by decreasing intensity 

 of folding at the nuclei or centers appears to have been characteristic of 

 mountain formation during certain great time units, such as the Archeo- 

 zoic, Proterozoic, Cambro-Ordovician, later Paleozoic, Mesozoic, and 

 Cenozoic. 



Structure of existing Mountains 



comparison of ranges 



It is not many years ago that the main mountain ranges of the globe 

 were considered to be of the fold type, and, moreover, that the age of 

 each individual system, much as it exists today, was considerable. A 

 statement by Gilbert,'^ and quoted by W. M. Davis in ^'Explorations in. 

 Turkestan, 1905," is interesting in this connection : 



"In the Appalachians corrugation has been produced by folding, exception- 

 ally by faulting ; in the Basin Ranges commonly by faulting, exceptionally by 

 flexure." 



All mountain ranges examined by the writer, considered as to their 

 general form and height, appear to be no older than the close of the 

 Tertiary, and to be of the plateau type, with warped or faulted margins. 

 With regard to Gilbert's statement above, it may be remarked that the 

 present Appalachians appear really to be variants of the Cordilleran type, 

 the quantitative result being much less in the case of the former, whereas 

 the folds seen in the slopes of the Appalachian plateaus, as they exist 



^ G. K. Gilbert : Washington, 1875, pp. 01, 62. 



