274 L. V. Pirsson — Classification of^ Igneous Rocks. 



strikingly to a group of the neplielite-syenite family 

 occurring in the Christiania region.^ ^ These ideas of 

 Rosenbusch and Brogger respecting the genetic origin 

 of rock-groups, as well as the family relationships of 

 rocks over particular areas, comprised by Iddings^^ 

 under the term consanguinity, have had a wide influence, 

 not only on classification, but on the development of 

 petrology.^ ^ 



The geologic-petrographic classification has also been 

 used in England by Harker^^ who divides the rocks into 

 plutonic, hyp abyssal, and volcanic. They are then sub- 

 divided according to their petrographic characters. On 

 the other hand Teall had earlier pronounced for a purely 

 petrographic classification, based primarily on composi- 

 tion and texture.^ ^ In none of these systems is there any 

 distinct recognition of the quantitative relation of vari- 

 able minerals within a group. 



Remarks on ike Geologic-petrographic System. — Brogger evi- 

 dently finds difficulty in usmg the geologic characters as a means 

 of discriminating, classifying, and naming different kinds of rocks, 

 since he remarks ... "a rhombic porphyry (a variety of syenite 

 porphyry) is to use a hypabyssal rock type, indifferently whether 

 it occurs as a dike rock as border facies of angite syenite, or even 

 as a lava flow."^^ He also says, relative to the dependence of 

 texture on the rate of cooling — ''But whether this slow cooling 

 (which yet in comparison with the extremely slow cooling in the 

 central part of a laccohth is relatively more rapid) has taken 

 place along the border portion of an abyssal magma, in a very 

 large dike fissure, or finally in the more central part of a very 

 thick extension, must be regarded as quite irrelevant, if, even- 

 tually, the consolidation has furnished practically the same rock 

 with essentially the same texture and composition."^^ 



The student is desired to note this because it shows clearly the 

 struggle to unite in one and the same classification very different 

 features that relate to quite different things. As an instance of 

 what happens when we use geologic position and petrographic 

 characters in the same classification to determine a kind of rock 



^° Das Ganggef olge des Laurdalits ; Eruptiv. Gest. Kristiania-gebietes ; 

 Videnskab. Skrift. M. N. KL, 1897. 



^^ Origin of Igneous Eocks. Bull, Phil. Soc. Washington, vol. 12, p. 128; 

 3892. 



" See also Daly, E. A. Igneous Eocks and their Origin; p. 311; 1914. 



"Petrology for Students, 3d Ed. 1902; p. 22. 



" British Petrography, 1888, p. 69 et. seq. 



^^ Grorudit-Tinguait-Serie, quoted ante, p. 123. 



^« Ibid. p. 124. 



