BY E. C. ANDREWS. 799 



residuals of the earlier gradation periods survive to-day, the 

 oldest as peaks, the next in point of age as ridges and subhorizontal 

 masses. Well into the canon cycle, however, the southern 

 portion of New England was deluged with successive basalt flows."^ 

 Alternating hard and weak structures characterised the products 

 of this volcanism; some of the flows were dense and offered great 

 resistance to deca}^; others, again, by virtue of marked columnar 

 structure and composition, were unstable as shales. The result 

 is that even during such a brief geographical period as the later 

 canon cycle, the basalts have been "benched" back for many 

 miles in the form of huge terraces, the upper portion of each 

 surviving bench representing the surface (^upper) of a hard lava 

 sheet, the material lost to the volcanic rocks durincj this "terracing" 

 being redistributed by the streams to form the major portion of 

 the extensive "North-west" black-soil plains. The granite mesas 

 and buttes to the north, however, have preserved their outlines in 

 great measure during the whole of the canon cycle, thus laying claim 

 to being the " survival of the fittest." Loss there has been here 

 decidedly, but confined to the development of shallow valleys by 

 undercutting. 



Before the present cycle has advanced to late maturity, the 

 story of the great late Tertiary basalt deluge will survive only in 

 a series of dykes, necks and stray volcanic knobs in the central 

 area. Thus care must ever be exercised in differentiating between 

 monad nocks and later imposed conditions such as recent volcan- 

 icity. In all the endurance of residuals is evident. 



The enduring quartzites and hard Silurian rocks of which the 

 remnants of the Jenolan Plain consist represent the central and 

 most resistant masses in the Blue Mountain area to the attacks 

 of erosion, for the Jenolan Plain itself evidences a reduction of 

 hard and soft masses alike owing to the length of the cycle. 

 Thus during uplifts which are in the main very similar, the 

 centres of successive elevations being essentially coincident, the 



* The relative youth of these tlows is demonstrated from the fact that 

 they filled valleys excavated during the canon cycle. 



