113 
THE STRUCTURE OF ROCK MASSES 
(STRATIFICATION, JOINTS, CLEAVAGE). 
By DAVID FORBES, F.R.S., &c. 
[PLATE LVII.] 
I T appears extremely probable that one of the first observations 
which the beginner in Geology or Physical Geography would 
make, upon commencing in the field the study of the rock 
masses which compose the exterior of our globe, would be to 
notice that rocks are not mere amorphous or structureless 
aggregations of mineral matter, disposed at random like rubbish 
shot out of a cart, but that, as a rule, they possess some definite 
structure or internal mechanical arrangement of the particles 
of which they are built up ; a little further observation will 
then point out that such an internal arrangement may, fre- 
quently at least, be altogether independent of the external con- 
figuration of the rock masses themselves. 
It will next be perceived, that such rock structure may differ 
greatly in character in the different classes of rocks, and the 
conclusion will assert itself that, of all the varieties of structure 
met with in the field, those in which the particles show an 
arrangement in more or less parallel lines along which the rock 
can usually be more easily divided or split up than in other 
directions, are by far the most frequent in occurrence. 
A still minuter consideration of parallel structure teaches 
that it may be classified under five distinct heads, to which the 
terms stratification, joints, cleavage, foliation, and striation * 
* The term “striation ” or “vitreous striation” is here employed to signify 
the parallel arrangement which is entirely due to the development of what 
are called the “ striae ” of fusion, such as are seen in glass, furnace slags, 
obsidian, vitreous lavas, glacial ice, &c. This structure owes its origin to 
the effects of the (uuequal rate of?) movement in the different layers of 
a substance whilst in a more or less vis fid condition, in the act of consoli- 
dating from a state of liquidity, 
VOL. IX. — XP. XXXV, I 
