38 



INTERNAL STRUCTURE OF ROCKS. 



The prevailing colours of slate are bluish or greenish grey : it has 

 a silky lustre. Slate rocks, have, frequently, a distinct slaty structure 

 and may even be split in two directions, which have an acute angle 

 with each other; but some slate rocks have a compact structure, and 

 will not admit of splitting. Slate yields to the knife : it is fusible 

 into a black slag. The composition of slate is various : indeed, by 

 many geologists it is not regarded as an homogeneous rock. Its 

 composition has been given as under : — Silex 48, alumine 23, man- 

 ganese 1-6, oxide of iron 11*3, oxide of manganese 0-5, potass 4*7, 

 carbon 0*3, water 7*6. The quantity of carbon increases in the up- 

 per formations of slate, and it passes by a greater admixture of car- 

 bon, into a soft, dark, slaty bed, denominated shale by the English 

 miners. Slate is a very extensive formation, composing entire moun- 

 tains in many alpine districts. 



Basalt and compact lavas are classed, by some mineralogists, with 

 simple minerals, but they are composed of three or more simple 

 minerals closely united : — they will be afterwards described. 



Some of the minerals, here enumerated, compose entire rocks ; 

 other rocks are composed of an intermixture of two or more simple 

 minerals, either cemented together by another mineral substance, or 

 the minerals are crystallized and united without a cement. The 

 different modes in which simple minerals are found, united together 

 in rocks, have given rise to the following terms : — 

 Granitic^ composed of grains or crystals united without a cement, as 



in granites, and some sand-stones. 

 Porphyritic, composed of a compact homogeneous rock, in which 

 distinct crystals or grains are imbedded. The compact stone 

 is called the base, and sometimes the paste. The base of some 

 porphyritic rocks is granitic ; in this case, some of the crys- 

 tals are much larger than the rest. 

 Amygdaloidal, containing rounded or kernel-shaped cavities, filled 



with mineral matter of a different kind. 

 Breceia is composed of angular fragments of rocks, cemented to- 

 gether. 



Pudding-stone consists of rounded stones imbedded in a paste.* 



Fragments of stone, broken from simple rocks, display the struc- 

 ture of the internal parts. The face of the broken part is called the 

 fracture. This internal structure may be denominated the mineral 

 structure, and is either 



Compact, without any distinguishable parts or divisions; or 

 Earthy, composed of minute parts resembling dried earth. 

 Granular, composed of grains. 

 Fibrous, composed of long and minute fibres. 



* When fragments of stone, whether angular or rounded, are large and are 

 imbedded in strata of indurated clay, sand or sandstone, they are called Conglom- 

 erates. 



