70 



The same authors recognise three varieties of each of the principal types. 

 The varieties of the granitic type depend on the mutual relations of the 

 constituent minerals. They are (1) the granitic type proper, (2) the pegmatitic 

 type, and (3) the ophitic type. (1) The varieties of the trachytic type depend 

 upon the character of the groundmass. They are (1) the petrosiliceous or 

 felsitic type, (2) the microlitic type, (3) the vitreous type. 



It will be observed that the terms granitic and tracln/tic as used by the 

 French petrographcrs are not synonymous with the terms granular and 

 porphyritic.' Granitic includes the coarsely- granular and granito-porphyritic 

 textures. Trachytic includes the finely granular, felso-porphyritic and vitro- 

 porphyritic textures. 



The following is the system of classification adopted in this work for 

 convenience of description. It is essentially based on the systems of 

 ROSEXBUSCH and Messrs. Fouauls and Lfivy ; but of course no distinction is 

 made between rocks of the same composition and texture merely because they 

 have been produced at different periods. The classification is essentially a 

 mineralogical one ; but the rocks are arranged as far as possible according 

 to BUXSEX'S law. 



GROUP A. 



ROCKS COMPOSED OF THE FERRO-MAGXESIAX MINERALS : OLIVIXE, EXSTATITE, 



ATJGITE, HORXBLEXDE AXD BIOTITE. FELSPAR ABSEXT J OR, IF PRESENT, 



OCCURRIXG OXLY AS AX ACCESSORY COXSTITL 1 EXT. 



With Olivine. 

 I. OLIVIXE ROCKS. Dunite (HoclisteUer)^ 



II. OLIVIXE-PYROXEXE ROCKS. 



Olivine-enstatite rocks. Saxonite 



Olivine-enstatite-augite rocks. (Lherzolite. De la MetherieJ . 



Olivine- augite rocks. Picrite (Tscliermalt) J 



(1) The ophitic texture is common to plutonic and volcanic rocks. 



(2) Z.D.G.G., 1864, p. 341. 



(3) Lithological Studies. Cambridge, Mass., 1884, p. 85. 



(4) Theorie de la Terre, vol. II., p. 281. Paris, 1797. 



(5) Die Porphyrgesteine Osterreichs. Wien, 1869. p. 241. The picrite of Tschermak is 

 intrusive in cretaceous rocks. Giimbel (Die palaeolithischen Eruptivgesteine des Fichtelgebirges. 

 Miinchen, 1874, p. 38.) has proposed the term palaeopicrite for a rock of similar composition, 

 which is associated with palaeozoic sediments in the Fichtelgebirge. 



