118 A. W. Jargon — Xo-mtn'-h.tt»r> of (',■//*/</// ;„<> Books. 



physical conditions of temperature and pressure that have de- 

 *tion and that, equally with the 

 original magma, have determined the precise minerals which we 

 now find in the rock itself. It is true we do not yet know 

 enough of the relations between varying conditions of tempera- 

 ture and pressure, and the resulting minerals to be able to 

 reason backward from the minerals themselves to the conditions 

 of which they are the (partial) expression; some progress has 

 been made in this direction and certainly more is to be ex- 



The mineral composition of a rock will give us a conception, 

 then, not only of the original chemical composition of the mag- 

 ma from which it was derived, but also (particularly with 



ft-s/o,*, see p. i'.'i of the nhvsie;il conditions which have pre- 

 vailed during the process by which the rock acquired its present 

 . jests both chemical and physical conditions 

 while the ci .--.rests only the former. Whence 



I conceive that the use of chemical facts in constructing rock- 



The inconvenience of making a quantitative chemical analysis 

 before deciding the name- of a rock is certainly not the least ob- 

 jectionable feature of the plan. 



Geology must always remain more or less of a natural history 

 science and the field geologist will always need a name that 

 will convey some conception of the a/>pe<irance of a rock, such 

 as ;; mineralogical name will give him. 



2. Geological The geological facts that might be used in 

 this connection are form, age and origin. Form has been used 

 to some extent, but it is now so generally recognized that the 

 same rock can occur as dyke, bed, sheet, stream, or stock, that 

 it has properly ceased to have any significance so far as name is 

 concerned. Not so, however, with age. The continental geol- 

 ogists divide the massive crystalline rocks into two classes 

 which they call '-older" or "younger" according as the rock 

 was formed before or after the inception of the Tertiary period ; 

 and based upon this distinction, the same mineral agj 

 provided with two different names according as it is of Pre- 

 Tertiary or Post-Cretaceous age. Thus I have held in my band 

 two specimens that could not be distinguished from each other 

 even in thin sections under the microscope. Both contained 

 quartz and sanidine porphyritically developed, the ground- 

 mass of both was crypto crvstaliii e thro ighout and yet the one 

 was "quartz-porphyry" because Pre-Tertiary. and the other 

 was "quartz- trachyte" because Post-Oeiuceoiis. For the same 

 reason melaphyr is separated from basalt, with which it i* 

 otherwise identical ; hornblende porphvrite from hornblende 



