82 STRUCTURAL AND FIELD GEOLOGY 
another. It is the fine-grained and compact stones, in which the naked 
eye may fail to recognise the nature of the component ingredients, that 
chiefly puzzle him. How are the various compact igneous rocks to be 
distinguished from each other, or indeed from other compact rocks of 
any kind? One may, of course, with the help of a pocket-lens, be able 
to detect the character of some fine-grained rocks, but in the case of 
many others a careful microscopical examination of thin slices wiil be 
absolutely necessary before one can be sure of their nature. It must be 
admitted, indeed, that we cannot know all about rocks, whether they be 
coarse-grained or fine-grained, before we have submitted them to careful 
scrutiny under a microscope. The student who does not desire to 
specialise as a petrologist, however, may nevertheless come to be very 
knowing in the matter of rock-determination by following a few simple 
rules or methods. 
In attempting to determine a rock in the field, the beginner should 
be careful, in the first place, to examine fresh surfaces. Rocks which 
have been exposed to the weather are always more or less altered, the 
weathered crust being thick or thin, according to the nature of the rock, 
and the length of time it has been subject to the mechanical and chemical 
action of the superficial agents of change. It need hardly be said, there- 
fore, that the fresh rock may differ very much from the crust which 
covers it. Assuming, then, that a fresh surface has been obtained, the 
student endeavours to ascertain to which of the great classes the rock 
belongs—is it crystalline or fragmental? If it be coarse-grained, he 
should have no difficulty in answering this question. Should it be 
crystalline, the crystalline ingredients will either be confusedly aggregated 
or arranged in parallel lenticular folia. In the former case it is probably 
an igneous rock* ; in the latter case it is aschist. If, on the other hand, 
the rock be composed of rounded waterworn materials, it is a conglomerate 
of some kind ; but if the included fragments be angular, it is a breccia. 
It is obvious, therefore, that in the case of coarse-grained rocks the first 
step towards determining them is simple enough. With finer-grained 
rocks, however, it is otherwise. At first the observer may have some 
difficulty in discriminating between the mineral constituents of such 
rocks even with the help of a lens. It is a good plan to pound a 
chip of the fresh rock with the hammer, and reduce it by rubbing to a 
gritty powder. In the case of crystalline igneous rocks and many schists, 
this process often succeeds in separating the rock-constituents from each 
other more or less completely, so that they can be turned about with 
the point of a knife in all directions, and subjected to a more thorough 

* It might be, however, coarsely crystalline limestone, dolomite, or 
anhydrite. These rocks are s¢mple—z.e. they are composed throughout 
of one mineral ingredient, and could readily be determined by the simple 
tests mentioned on pp. 28, 29. A coarsely crystalline igneous rock, on 
the other hand, would usually be recognised by its composite character, 
and by the fact that its constituent minerals were not apparently affected 
by dilute acid. 

