26 
NATURE STUDY. 
The Study of Rocks. 
* 
BY GEORGE I. HOPKINS. 
A thorough acquaintance with individual minerals, in small 
particles as well as large ones, is the best equipment for the 
study of rocks composed of two or more of those minerals. In 
a former article a few simple and easily applied tests were 
given for the recognition of the three most common minerals in 
New England, viz., quartz, feldspar, and mica. Any one who 
has once seen specimens of the last named, whether in small 
scales or large sheets, whether black, white, pearly, or cop¬ 
pery in color, would rarely fail to recognize it afterward where- 
ever met with, and in whatever combination. The quartz, too, 
is generally easy of recognition, as was said before, by its lack 
of cleavage and its hardness. The members of the feldspar fam¬ 
ily all resemble one another closely in their general appearance, 
luster, color, hardness, etc., but there are two groups of this 
family that the student must learn to distinguish from each 
other because it is essential in the classification of certain rocks. 
In one group there are two cleavage planes perpendicular to each 
other, and in the other the cleavage planes are all oblique to 
each other. These are named orthoclase and plagioclase re¬ 
spectively. There are several varieties of the latter, the most 
important of which are albite, oligoclase, labradorite, and anor- 
thite ; but as they differ chiefly in their chemical composition 
the ordinary student would be unable to distinguish them. Af¬ 
ter a few specimens have been carefully examined there will be 
little difficulty in detecting the feldspar. It will require a some¬ 
what more careful examination to determine whether it be or¬ 
thoclase or plagioclase, but as the former is much more com¬ 
mon, the probabilities are in favor of any given specimen being 
orthoclase. It might also be stated that although all the feld¬ 
spars are sometimes found to be white in color, yet albite is 
