65 



and mode of occurrence. (1) Thus, the ophitic plagioclase-augite-olivine 

 rock of Tideswell Dale, for example, which is associated with carboni- 

 ferous strata would be termed an olivine-diabase ; whereas rocks 

 having absolutely the same composition, and micro- structure from 

 Tobermory (Mull), Portree (Skye) and Iceland, which are of Tertiary age, 

 would be termed dolerites. Again, the rock with a somewhat resinous lustre, 

 containing porphyritic crystals of labradorite, hypersthene and augite, 

 embedded in a microlitic groundmass, from Carhope on the Coquet, would 

 be termed enstatite- or hypersthene-porphyrite, because it is of old-red- 

 sandstone age. The same rock from St. Egidi (Steiermark), the Rocky 

 Mountain region of North America, and the Andes would be termed enstatite- 

 or hypersthene-andesite ; the only difference being that in the last-mentioned 

 localities the rock in question is of Tertiary age. (2) English, and, to a certain 

 extent, also American petrographers, (3) do not accept this principle, and, as 

 a natural consequence, great confusion arises in petrographical nomenclature. 



While declining to accept geological age as a primary factor in classifi- 

 cation, in common with all British petrographers, the present writer is 

 strongly of opinion that, if possible, it should receive indirect expression. 

 This position may be illustrated as follows. In deciding on the relative 

 importance, for classificatory purposes, of the characters presented by fossils, 

 the palaeontologist lays special stress on those which stand in any way 

 related to the organisation of the living animal. Minute peculiarities are, 

 on this account, often of more value than others which are relatively 

 conspicuous. So, when any peculiarity of texture or composition can be 

 shown to characterise rocks of a particular period, that peculiarity should 

 be utilised for purposes of classification. In precisely the same way 

 facts with regard to origin, mode of occurrence and even geographical 

 distribution may receive indirect expression in classification. 



If this point of view be accepted then it follows that petrographical 

 classification should be based primarily on facts which can be ascertained by 

 the examination of hand specimens ; and that facts with regard to origin, 

 age, mode of occurrence and geographical distribution should only receive 

 expression in classification in so far as they stand related to peculiarities of 

 texture and composition ; at the same time the greatest importance should be 

 attached to the former class of facts in deciding on the relative value of the 

 latter. 



The Continental system of classification is of course based on the real or 

 supposed fact that peculiarities of texture and composition stand in direct 

 relation to geological age. The only author who appears to have treated this 

 subject in a systematic way and from a modern petrographical stand-point is 



FRENZEL. (4) 



(1) ROSENBUSCH. Mikroskopische Physiographie der massigen Gesteine. 



(2) It will be understood that these cases are merely quoted for the purpose of emphasising 

 the argument. Others might be selected. 



(3) WADSWOBTH. Classification of Rocks. Bull. Mus. Comp. Zool. Massachusetts. 1879. 

 BUTTON. Geology of the High Plateaus of Utah. 1880. 



(4) Uber die Abhangigkeit der mineralogischen Zusammensetzung und Structur der 

 Massengesteine vom geologischen Alter. Zeit. f.d. Ges. Naturw. Band LV. 1882. p. 1. 



