PART I. CHAPTER VII. 



95 



How to distinguish Rocks Basalt. 



hornblende, and carbonate of lime. This knowledge cannot be 

 acquired from books, but requires personal inspection, and the 

 aid of a teacher. It is well to accustom the eye to know the 

 appearance of rocks under the lens. To learn to distinguish 

 felspar from quartz is the most important step to be first aimed 

 at ; when these occur in a granular and uncrystallized state, the 

 young geologist must not be discouraged if, after considerable 

 practice, he often fails to distinguish them by the eye alone. If 

 the felspar is in crystals, it is easily recognized by its cleavage ; 

 but when in grains the blow-pipe must be used, for the edges of 

 the grains can be rounded in the flame, whereas those of quartz 

 are infusible. If the geologist is desirous of distinguishing the 

 three varieties of felspar above enumerated, or hornblende from 

 augite, it will often be necessary to use the reflecting goniometer 

 as a test of the angle of cleavage, and shape of the crystal. 

 The use of this instrument will not be found difficult. 



The external characters and composition of the felspars are 

 extremely different from those of augite or hornblende ; so that 

 the volcanic rocks in which either of these minerals decidedly 

 predominate, are easily recognized. But there are mixtures of 

 the two elements in every possible proportion, the mass being 

 sometimes exclusively composed of felspar, at other times solely 

 of augite, or, again, of both in equal quantities. Occasionally, 

 the two extremes, and all the intermediate gradations, may be 

 detected in one continuous mass. Nevertheless there are certain 

 varieties or compounds which prevail so largely in nature, and 

 preserve so much uniformity of aspect and composition, that it 

 is useful in geology to regard them as distinct rocks, and to as- 

 sign names to them, such as basalt, greenstone, trachyte, and 

 others, already mentioned. 



Basalt. — As an example of rocks in which augite greatly 

 prevails, basalt may first be mentioned. Although we are more 

 familiar with this term than with that of any other kind of trap, 

 it is difficult to define it, the name having been used so vaguely. 

 It has been very generally applied to any trap rock of a black, 

 bluish, or leaden-grey colour, having a uniform and compact 

 texture. Most strictly, it consists of an intimate mixture of 

 augite, felspar, and iron, to which a mineral of an olive green 

 colour, called olivine, is often superadded, in distinct grains or 

 nodular masses. The iron is usually magnetic, and is often ac- 

 companied by another metal, titanium. Augite is the predo- 

 minant mineral, the felspar being in much smaller proportions. 

 There is no doubt that many of the fine-grained and dark- 

 coloured trap rocks, called basalt, contain hornblende in the 

 place of augite ; but this will be deemed of small impprtance 



