8g Eee GEOGNOSY. ee it 
Muddy (pelitic), having a texture like that of dried mud. Cryptoclastee 
or compact, where the grains are too minute to reveal to the naked 
eye the truly fragmental character of the rock, as in fine mudstones 
and other argillaceous deposits. 
Granular, composed of worn grains or of irregular crystallme 
particles, as in dolerite, granite, sandstone and marble. This texture 
may become so fine as to pass insensibly ito compact. The 
crypto-crystalline portions of some igneous rocks, where the com- 
ponent ingredients cannot be determined except with the micro-_ 
scope, are sometimes called aphanitee. 
Massive, unstratified, having no arrangement in definite 
layers or strata. Lava, granite, and generally all crystalline rocks | 
which have been erupted to the surface, or have solidified below 
from a state of fusion (or plasticity), are Massive rocks. oe 
| Stratified, bedded, composed of layers or beds lying parallel 
to each other, as in shale, sandstone, limestone, and other rocks — 
which have been deposited in water. Laminated, consisting of fine 
leaf-like strata or lamine; this structure being characteristically 
exhibited in shales, is sometimes also called shaly. 
Foliated, consisting of minerals that have crystallized in 
approximately parallel lenticular and usually wavy layers or folia. 
Rocks of this kind commonly contain layers of mica, or of some 
equivalent readily cleavable mineral, the cleavage planes of which 
coincide generally with the planes of foliation. Gneiss, mica-schist 
and tale-schist are characteristic examples. So distinctive, indeed, 
is this structure in schists, that it is often spoken of as schistose. In 
gneiss it attains its most massive form; in chlorite-schist and some 
other schists it becomes so fine as to pass into a kind of minutely 
scaly texture, often only perceptible with the microscope, the rock 
haying on the whole a massive structure. 
Fibrous, consisting of one or more minerals composed of 
distinct fibres. Sometimes the fibres are remarkably regular and 
parallel, as in fibrous gypsum, and veins of fibrous aragonite or 
calcite (satin-spar); in other instances, they are more tufted and 
irregular, as in asbestus and actinolite-schist. 
Streaked, having some or all of the component minerals 
arranged in streaky lines, either parallel or convergent, and often 
undulating. This structure, conspicuously shown by the lines of flow 
in vitreous rocks like obsidian, is less marked in such crystalline - 
rocks as diorite and dolerite. It can be seen on a minute scale, 
however, in many crystalline masses when examined with the micro- 
scope. (See Fluxion-structure, p. 104.) 
Cavernous (porous), containing irregular cavities due, in 
most cases, to the abstraction of some of the minerals; but oceasion- 
ally, as in some limestones (sinters), dolomites and lavas, forming part 
of the original structure of the rock. 
Cellular.—Many lavas, ancient and modern, have been 
saturated with steam at the time of their eruption, and in conse- 
quence of the segregation and expansion of this imprisoned vapour, 

