16 MB. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April I914, 



ortkoclase; the latter mineral occurs in Carlsbad twins, and is 

 very turbid, showing decomposition into kaolin and sericite. 

 Plagioclase showing albite-twin laruellation, rare. 



Tourmaline abundant, in large crystals, with blue and brown 

 pleochroism. Some biotite and a little muscovite. 



Magnetite is fairly abundant. Epidote and chlorite occur 

 sparingly. 



( A 33 ) T o u r m a 1 i n e - v e i n ; Jamiraya , Lluta h E-i ver. 



Microscopic characters: An irregular mosaic of quartz, 

 orthoclase, and plagioclase, with abundant tourmaline. 

 Topaz is fairly abundant in colourless prisms. 



Epidote occurs sparingly; titaniferous magnetite (altering 

 to leucoxene) and pyrite are present, as also a little secondary 

 calcite. 



In an igneous intrusion of huge dimensions, such as a mountain- 

 core of this nature, which has been modified probably both by 

 differentiation and assimilation, it is obviously impossible to select 

 one or Wo specimens for description or analysis as typical of the 

 whole mass, and to which a definite name can be given. Some of 

 the more acid examples with associated tourmaline-veins would 

 undoubtedly be assigned to the granites or adamellites; 

 Avhile those with quartz sparingly developed and abundant ferro- 

 magnesian minerals would probably be found to approach in 

 composition the tonalites or diorites. 



The general composition of these so-called ' Andes granites ' has 

 been described 1 as that of a grano diorite; and, although 

 in the specimens from my collection the plagioclase does not 

 appear to predominate very largely over the orthoclase, the 

 name may conveniently be retained until further chemical 

 analyses can be made. It is, however, used here not as a definite 

 statement of the alkaline content, but merely as indicating the 

 general facies of the rock, which ranges from the granites and 

 granodiorites to the tonalites. 



All theories as to the origin of these deep-seated intrusions of 

 plutonic rock must, for the greater part, necessarily be based on 

 speculative assumptions ; and, since erosion has never penetrated 

 them very deeply, their field-relations are limited and difficult of 

 interpretation. Many of the leading authorities hold widely- 

 divergent views on the subject, and it is certain that the final 

 explanation will only be arrived at by the constructive com- 

 bination of a vast number of isolated facts. 



The term ' batholith ' was introduced by Prof. Eduard Suess to 

 describe those large granite-intrusions which so often form the 

 core of great mountain-chains. They may be defined as shield- 

 shaped masses of plutonic rock, intruded as the result of fusion 

 of older formations (durchschmelzungsmasse), and growing 

 broader to unknown depths. They usually penetrate the crust 



1 0. Nordenskjold, ' Die krystallinisehen Gesteine der Magellansliinder ' 

 Svenska Exped. till Magellan slandern, Stockholm 1901, vol. i, p. 175. ^ 



