84 THE CANADIAN NATURALIST. [Vol. ix. 



6. Rocks consistijig of a triclinic feldspar and mica. — The 

 term dioryte, formerly defined as a rock consistiog of oligoclase or 

 albite and hornblende, has been introduced into the the name of 

 a series of rocks containing no hornblende, but mica instead. 

 Thus : *' mica dioryte " is defined as a '' plagioclase-mica rock " 

 in which mica is substituted partly or wholly for hornblende, and 

 it is called mica-dioryte whichever of the triclinic feldspars be 

 present, even if anorthite. This change in the use of the name 

 dioryte so as to include a rock containing no hornblende, makes 

 " plagioclase " the essential constituent, and places mica and 

 hornblende in a subordinate position, as the heads only of sub- 

 divisions. 



The remarks made respecting syenyte apply equally here ; and 

 also those respecting " plagioclase." A mica-dioryte is like 

 granite, eminently an alkali-yielding rock, the mica (biotite) 

 aflFording usually ten per cent of potash ; and as granites often 

 contain oligoclase as well as orthoclase, the amount of potash 

 and soda in a "mica-dioryte" and a granite may not be very 

 widely diflferent. Dioryte, on the contrary, is prominently a 

 hornblende rock. 



Looking: to the mineralo2:ical and chemical constitution of the 

 rocks, we are naturally led to recognize alongside of a mica and 

 potash-feldspar series, which is headed by granite, also a mica and 

 soda-lime feldspar series, and to include in the latter the so-called 

 mica-diorytes. 



7. Hornhlendic or Avgitic. — Hornblendic and augitic rocks 

 stand apart as a general thing in all systems of lithology. Yet 

 the minerals are essentially identical in chemical composition, and 

 related in crystallization, though difi'erent in their occurring crys- 

 talline forms and in the angle of the cleavage prism. The identity 

 in composition is so close that chemical analysis is not able to 

 distinguish them. Hence the related eruptive rocks of the horn- 

 blendic and augitic series (or those containing the same species of 

 feldspar iu like proportions) must have originated in material of 

 essentially the same chemical composition. The relation between 

 the two minerals is thus far closer than between the triclinic species 

 of feldspars. 



Nevertheless, too much importance is not given them when each 

 is made distinctive of an independent series of rocks; for the very 

 wide extent to which augitic rocks retain unvaryingly their augitij 

 characters — such rocks constituting full two-thirds of the earth's 



