KINDS OF ROCKS. 425 



distribution of each, that makes the case like that of tra- 

 chyte and felsyte. The author has proposed to use the pre- 

 fix meta for metamorphic kinds when a rock occurs both 

 metamorphic and eruptive ; but this is not intended to indi- 

 cate a distinction in hind, but only to abbreviate the qualify- 

 ing word metamorphic. 



According to the principles above stated, a rock having 

 oligoclase or albite as its feldspar constituent cannot rightly 

 have the same name with one having either of the basic 

 feldspars, labradorite or anorthite, as an essential part, 

 although these feldspars are all embraced under the decep- 

 tive title of plagioclase (p. 275). Between anorthite and 

 oligoclase there is a difference of 20 per cent, in the silica, 

 and the former is simply a lime feldspar ; and the contrast 

 is large also between labradorite and oligoclase. Again, for 

 a like reason, as already explained (p. 411), a mica-bearing 

 rock containing little or no hornblende cannot properly be 

 classed with hornblendic rocks. 



3. It has been supposed that pre-Tertiary crystalline rocks 

 differed so decisively from the Tertiary and more recent, 

 that those of the two series should not bear the same name. 

 But geology knows nothing of any epoch of sudden transi- 

 tion in the mineral nature of eruptive rocks at the com- 

 mencement of the Tertiary era ; on the contrary, it shows 

 that the kinds made before and after this epoch are alike 

 in mineral constitution, and differ not always even in tex- 

 ture, but only in the greater prevalence after the Tertiary of 

 volcanic or subaerial ejected masses, and therefore of rocks 

 of the texture this involves. The distinction of doleryte 

 from diabase, with others similar, is of this chronological 

 kind. Rocks, like other objects in science, should evidently 

 be named from what they are, and not from the age in 

 whinh they may have been made. 



4. Since quartz is the most abundant of all the minerals 

 of the globe, it is the least characteristic of the ingredients of 

 compound rocks. Eecent lithologists have made it, in sev- 

 eral cases, distinguish only a section under a Tcind of rock. 

 Thus, there are dioryte and quartz-dioryte, felsyte and 

 quartz- felsyte, trachyte and quartz-trachyte. On the same 

 principle there are syenyte and quartz-syenyte, as adopted 

 beyond. 



5. The division of crystalline rocks into acidic and basic 

 rocks is explained on p. 274. The acidic afford on analysis 



