1870.] MACFARLANE— -ON CRYSTALLINE ROCKS. 307 



at all. These rock species are syenite, melaphyre and andesite, 

 which respectively represent the neutral development of the 

 granular, porphyritic, and trachytic orders of original rocks. If, 

 from among the syenites, melaphyres and andesites which have 

 been subjected to analysis, we select those whose oxygen ratio 

 best corresponds to neutrality, we have the following : — 



oxy^^nor 0:y.enor ^Sll^^H, ■ 



1. Syenite from the Stcilen Stiege, 



intbe Hartz,— Fachs.......". . 1 1848 56-30 



I r. Syenite from Monte Margohi, 



near Fredazzo, — KJerulf 1 2 2'2d oS^Oo 



lir. Svenite from the Schonberirer 



Thai in the JJergstrasse, — G. 



Bischof ] -i-OaJ 58-90 



IV. Syenite from Plauenscheu 



Grund, near Dresden,— Zirkel. t 2*288 59.83 



Average I 2-104 .58.28 



I. Mehiphyre from Schueidmiil- 



lorsberg, in llmeuthal, near 



llmeuau, — Yon Kichthofen.. . 1 1-938 .■')5-51 



ir. Khombic porphyry of Vetta- 



kolleu, classed with the mela- 



phvres, bv Xaumaun, Delesse 



Kjerulf. .^ I 2-017 50-— 



ill. Melaphyre from Bahrethal, 



near llfeld,— Streng 1 2-01 1 56-22 



1\'. Melaphyre from Leuchtburg, 



in the Thurimriau Forest, — 



Sochting \' 1 2-133 59-18 



Average 1 2-0-24 .50-73 



II. Augiiic andesite from Lowen- 



Inn-g, in Siebeugebirge, — Kjerulf 1 1-868 55-6'5 



II. H(n-nblendic Andesite, from 



Mcrapi, in .Java, — Frolss 1 1-975 57-60 



III. llorablendic .Vudesite from 



Stary Swietlan, — Tschermak. 1 2-091 56-92 



IV. Hornblendic Andesite, from 

 Stenzelljerg, in Siebengebirge, 



— Kammelsberg .". . . 1 2-332 -59-22 



Average 1 2 -060 57-85 



It would seem therefore from these iigures, that those rocks 

 which, in eompo.sition, are neutral or monosilicates, contain an 

 amount of silica averaging 57.62 per cent. 



As in chemistry we have acid salts, in which one atom of base 

 is combined with more than one atom of acid, so in litholosy we 

 have rocks in which the silica is present in much larger quantity 

 than is required for monosilicates. A very well defined series of 

 rocks is known in which the silica is present in such excess as to 



