

10 STRUCTURAL AND FIELD GEOLOGY 
microscope. Two series of felspars are recognised—one 
of these crystallising in monoclinic and the other in 
triclinic forms. The monoclinic series includes Orthoclase 
and Sanidine, while the triclinic class is répresentedmeby 
Microcline, Anorthoclase, and Plagioclase—the last-named 
forming a group of felspars which are all more or less closely 
related, and often hardly to be distinguished from each other 
without careful microscopical or chemical examination. As 
a group they are more or less readily differentiated from the 
monoclinic felspars by the inclination of their cleavage-planes— 
in the monoclinic felspars these planes being directed at right 
angles to each other, while in the triclinic group referred to 
they are not at right angles. Hence we have two series of 
felspars—namely, (a) Orthoclase, with rectangular cleavage, 
and (4) Plagioclase, with oblique cleavage. 
If felspars always assumed their external crystalline form and were of 
sufficient size, it would not be hard to distinguish between orthoclase and 
plagioclase. As rock-constituents, however, they are often so un- 
symmetric in shape, or occur as granules so small in size, that the geolo- 
gist must have recourse to other differentiating characters to distinguish 
between one felspar and another. Under the microscope, the plagioclase 
felspars can usually be recognised by their “multiple twinning.” A 
crystal or crystalline granule having this structure appears as if it were 
composed of a series of parallel plates or lamellae. 
Twinning is best seen in polarised light—each 
plate being differently coloured from its neigh- 
bours, so that a section of the mineral, if cut in 
the proper direction, exhibits a banded or striped 
appearance (Plate II. 1, 2). As the mineral con- 
stituents of a crystalline igneous rock usually lie at 
different angles and in different directions, it does 
not often happen that the structure referred to is 
not revealed by one or more of the individual 

i plagioclase crystals, which are exposed in the field 
of vision under a microscope. Even when the 
minerals are arranged in approximately parallel 
Fic. 3. — CRYSTAL 
layers, as in the case of schistose rocks, it is always 
possible to cut sections both in the direction of, and 
across, or at any angle to, the planes of foliation. 
Not infrequently the twinned structure can be seen by the naked eye or 
with the aid of a pocket-lens, when the felspars are fresh and not too small. 
The structure is revealed by the appearance of fine parallel lines, with 
which the crystals are ruled or striated—the lines marking, of course, the 
junction of separate twin lamellz. The twinning of orthoclase felspars is 
OF ORTHOCLASE: 
CARLSBAD TWIN. 
