1881 .] 
281 
[Crosby. 
kinds of texture, the concise and systematic statement of which 
gives rise to a natural classification of textures. But a classifica- 
tion jwesupposes a general definition of texture, and this involves 
a determination of the relations of texture to other classes of 
rock-structure. Now the structures of rocks are conveniently 
and naturally divided into three grades or classes, as follows : 
first, microscopic structures, for the study of which thin sec- 
tions are usually employed ; second, textures, which may be 
studied to the best advantage in hand-specimens ; and, third, 
those larger structural features of rocks, such as stratification, 
folds, faults, joints, etc., which can be observed, as a rule, only in 
large masses, and to which the term structure , when used without 
qualification, is commonly restricted. The main point to be noted 
here is that textures are a class of rock-structures which are essen- 
tially distinct from the microscopic structures on the one hand, 
and from the petrologic structures on the other. It is not 
intended, however, to assert the existence of sharj) lines of divi- 
sion between these different classes of rock-structure, for abrupt 
transitions are unknown in geology. 
The following definition of texture is in conformity with the 
foregoing and forms the basis of the classification of textures 
explained beyond. 
Texture is a general name for those smaller structural features 
of rocks which can be studied in hand-specimens , and which 
depend upon the forms and sizes of the constituent particles of 
the rocks and the ways in which these are united. 
By “ constituent particles” are meant, not the molecules of mat- 
ter composing the rocks, but those particles or masses, usually of 
sensible size, the coming together or development in association 
of which has made the rock, as, for instance, the pebbles in con- 
glomerate, grains of sand in sandstone, crystals of quartz , feld- 
spar and mica in granite, etc. As indicated in the definition, 
textures may be classified according to the sizes, the forms, and 
the modes of arrangement, of the constituent particles. But of 
these three modes of division, that based upon size is the most 
fundamental, especially if we draw the line at the limit of 
visibility of the particles ; and here it is important to remem- 
ber that the texture is always essentially a macroscopic fea- 
