BY E. C. ANDREWS. 801 



provisionally as Lower Cretaceous or Jurassic — a great plain was 

 developed at sea-level. No idea can be formed in this locality as 

 to the amount of movement initiating the cycle of erosion which 

 resulted in the formation of this plain. Observations conducted 

 in the dense siliceous granites of northern New England, how- 

 ever, show that the Bolivia Plain, consisting of flat-topped masses 

 Aarying from 4,300 to 4,600 feet in height, resulted from the old 

 age gradation of a plateau about 700-1000 feet in height which 

 had been elevated at the commencement of the cycle. The 

 Jenolan peneplain is approximately the same height as the 

 Bolivia example and is probably its southern extension, although 

 the intervening areas of soft Palaeozoic strata have not been able 

 to survive the erosive activities of successive cycles, and thus the 

 relation can be inferred only from the general topographical 

 similarity of the two areas. 



The Jenolan period was one of long duration. Canons had 

 been cut in the plateau, mature valley S3^stems developed; these 

 in turn had expanded into wide plains under the action of lateral 

 corrasion, until, at the close of the cycle, a few rounded eminences 

 (PI. xxxix., a) only of the most durable Silurian and Devonian 

 rocks remained to attest to the existence of the old upland. The 

 rivers even in flood time pushed loads of silt only beyond their 

 mouths, and doubtless limestones were deposited in the clear water 

 off-shore. Naturally during such a period of stable equilibrium, 

 or rather one in which gradation and the algebraic sum of the 

 elevations and depressions resulted in the formation of a plane 

 near sea-level, the sea had encroached considerably on the land 

 surface. Immediately after the initial elevation it had built up 

 its off-shore base, then it had marched inland, destroying the bars 

 and piling the waste to form the continental shelf. As the bars 

 perished, the land was attacked, and the continental shelf grew 

 at the expense of the coast. Large cliffs were doubtless in this 

 case the expression of youthful sea attacks, but as the coast 

 became subdued Vj}- subaerial agencies the cliffs of youth gave 

 place to more subdued forms. As the sea encroached on the 

 land, wave-base became progressively less deeply seated, the sea 



