18 ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April 1914, 



in the case of the hornblende-crystals, which not only show 

 quartz eating its way in from the edges, but also colourless 

 patches in the centre of the sections devoid of pleochroism, 

 although the} r exhibit uninterrupted prismatic cleavage. Much 

 of the felspar must have belonged to the original rock, but it is 

 difficult to distinguish in this case the appearance due to corrosion 

 from that due to pegmatitic growth. The occurrence of such a 

 rock agrees well with Dr. Daly's statement that 



' If ... a crystallized granodiorite batholith be itself attacked by a later 

 abyssal intrusive and in large part stoped away and remelted, the secondary 

 magma collecting at the roof of the later batholith should be more acid 

 than [the] granodiorite.' (Am. Journ. Sci. ser. 4, vol. xxvi, 1908, p. 45.) 



From the hypothesis of overhead magma tic stoping, Dr. Daly x 

 has evolved a theoiy for the secondary origin of certain granites by 

 a process of differentiation, after assimilation of an acid cover by 

 a gabbroid magma. 



While agreeing with that author's views on the means by which 

 a plutonic magma makes its way into the overlying strata, I 

 2onsider that it is not advisable to attach too much importance to 

 this latter theory, so far as my own observations go. The law of 

 decreasing basicity seems to hold good for the whole body of the 

 magma, just as much as for the crystallization of its component 

 minerals, even in the absence of any assimilation. In the present 

 case, assimilation undoubtedly appears to have taken place, and 

 probably has largely affected the composition of the rock ; but I 

 can see no clear evidence for assuming the granite to be directly 

 derived from a gabbroid magma b} r a complex process of this 

 nature. It is true, however, that in Jurassic times the volcanic rocks 

 of the district were of an andesitic or basaltic nature, and their 

 plutonic equivalent may probably be regarded as the hypersthene- 

 norite of Ancolocalla ; while in post- Cretaceous times the intrusive 

 core had attained the composition of a granodiorite or granite, and 

 the Tertiaiy and recent lavas, to be described later (pp. 19 et seqq.), 

 have for the most part a pronounced acid character. 



Any connexion, however, between the basic magma of Jurassic 

 times and these Tertiary acid rocks must remain purely a matter of 

 speculation. 



(c) The Volcanic Rocks of the Western Cordillera. 



The Western Cordillera differs from the Eastern or Cordillera 

 Real in the fact that it is essentially a volcanic range formed of 

 numerous, more or less isolated, snow-capped, dormant and extinct 

 volcanoes, which frequently attain heights ranging between 19,000 

 and 20,000 feet. The great altitudes and inaccessibility of many 

 of these peaks have prevented investigators from studying them 

 in great detail, and the presence of a true crater is in most cases 

 purely a matter of conjecture. In the case, however, of such ideal 



1 Am. Journ. Sci. ser. 4, vol. xv (1903) p. 269 ; vol. xvi (1903) p. 107 ; 

 vol. xx (1905) p. 185 ; and vol. xxvi (1908) p. 17. 



