- Pan IT. § vii.] DETERMINATION OF ROCKS. 181 
are named from the predominant enclosed mineral, as chiasto- 
lite-slate, andalusite-schist, ottrelite-schist, &c. (p. 121). 
y. The rock is composed of a mass of ray-like or fibrous crystals 
matted together. If the fibres are exceedingly fine, silky, and 
easily separable, it is probably asbestus; if they are coarser, 
greenish to white, glassy, and hard, it is probably an actinolite- 
schist (p. 121.) 
6. The rock has a hardness of nearly 7, and splits with some difficulty 
along micaceous folia. It is probably a quartzose variety of 
mica-schist, quartz-schist, or gneiss (pp. 120-128). 
e. The rock shows on its weathered surface small particles of quartz 
and folia of mica in a fine decomposing base. It is probably a 
fine-grained variety of mica-schist or gneiss. 
_v. A fresh fracture shows the rock to have a fragmental (clastic) 
structure. 
Where the component fragments are large enough to be seen by the 
naked eye or with a lens, there is usually little difficulty in determining the 
true nature and proper name of the rock. 'T'wo characters require to be 
specially considered—the component fragments and the cementing paste. 
1. The Fragments.—According to the shape, size, and composition of 
the fragments, different names are assigned to clastic rocks. 
a. Shape.—lf the fragments are chiefly rounded, the place of the rock 
may be sought in the sand and gravel series (p. 156), while if they are 
large and angular, it may be classed as a breccia (p. 157). Some mineral 
-- substances, however, do not acquire rounded outlines, even after long- 
continued attrition. Mica, for example, splits up into thin laminz, which 
may be broken into small flakes or spangles, but never become rounded 
granules. Other minerals also which have a ready cleavage are apt to 
break up along their cleavage planes, and thus to retain angular contours. 
Cale-spar is a familiar example of this tendency. Organic remains 
- composed of this mineral (such as crinoids and echinoids) may often be 
noticed in a very fragmentary condition, having evidently been subjected 
to long-continued comminution. Yet angular outlines and fresh or little 
worn cleavage surfaces may be found among them. Many limestones con- 
sist largely of sub-angular organic debris. Angular inorganic detritus 
is characteristic of volcanic breccias and tuffs (p. 161). 
8. Size.—Where the fragments are hard rounded or sub-angular 
grains, the size of a pin’s head or less, the rock is probably some form of 
sandstone (p. 158). Where they range up to the size of a pea, it may be 
a pebbly sandstone, fine conglomerate or grit; where they vary from the 
size of a pea to that of a walnut, it is an ordinary conglomerate ; where 
they range up to the size of a man’s head or larger, it is a coarse con- 
glomerate. A considerable admixture of sub-angular stones makes it a 
brecciated conglomerate or breccia. 
y. Composition.—In the majority of cases the fragments are of 
quartz, or at least of some siliceous and enduring mineral. Sandstones 
consist chiefly of rounded quartz-grains (p. 155). Where these are 
unmixed with other ingredients, the rock is sometimes distinguished as a 
quartzose sandstone. Such a rock when indurated becomes quartzite 
(p. 126). Among the quartz grains, minute fragments of other minerals 
may be observed. When any one of these is prominent, it may give a 
name to the variety of sandstone, as felspathic, micaceous (p.158). Vol- 

