APPENDIX. 



201 



feet, and that this deposit might 



W 



O 





subside through 24,000 feet, and the bottom beds be still 

 subject to compression. Supposing accumulation 

 to have kept pace with the subsidence the result- 

 ing thickness would about correspond* with that of 

 the Alleghany formations. 



I will add a few examples of less known 

 mountain ranges. In 



JSJX^ ^ "* **" the Secti ™ P™ by 

 Dr. Hector (Quar. 

 .| Jour. Geol. Soc, Lond., Vol. XVII., 1861,) 

 of the Rocky Mountains east of British Colum- 

 bia, and from which Fig. 25 is taken, we 

 find some exceptional features of structure. 

 The central and highest region is also, rela- 

 tively to the north-east flanks, a region of up- 

 heaval, the lowest rocks appearing there at a 

 greater elevation. Moreover, relatively to this 

 central mass, we find well exhibited in the 

 rocks of the flanking ridges on the north-east 

 repeated folded flexures, of which, as in other 

 instances given, the axis-planes underlie towards 

 the centre of the chain. The central mass 

 itself is, however, neither comparatively nor 

 absolutely a region of contortion, fracture, or 

 intrusion ; the strata are but little disturbed, 

 and have a flat synclinal arrangement. On 

 the south-western flanks we find the lowest rocks 

 of all ; they are much folded, but in no de- 

 finite order, and at the base in the same 

 direction granite appears. This section has 

 several points of analogy with that of the 

 Alleghanies, the peculiar feature of the cen- 

 tral region being the chief discrepancy. The 

 author does not enter upon the discussion 

 of the order of formation of the features he 

 describes. 



The latest geological description of the Andes 



with which I am 



D. Forbes on the Andes of „„ _j_*„j • ,. , 



Peru acquainted is that 



of Mr. D. Forbes, 



'on the Andes of Peru,' (Quar. Jour. Geol. 



Soc, Lond., Vol. XVII., 1861) ; it is given as 



an emendation of that of D'Orbigny and Pissis. 



In the section of the Andes, as given by 



B 1 



