106 -  GEOGNOSY AND GEOLOGY. 
distinct crystallizations on the compact variety in a porphyritic manner. 
It contains various mineral substances not essential to its composition. 
The second variety is: 
Anhydrite, or anhydrous sulphate of lime. Of its mineralogical 
modifications only the scaly-granular, radiated, and compact, are of 
geoynostical importance. White prevails less, as a color, than grey and 
blue. Anhydrite becomes converted into gypsum by attracting moisture 
from the atmosphere. During this chemical action a considerable increase 
in volume takes place, by which whole masses are crumbled to pieces or 
shattered. 
B. Hereronomic Rocks. 
It has already been mentioned that by heteronomic rocks we understand 
those in which two principal parts are to be distinguished. The one 
consists of hard pieces, or fragments, the other of a generally earthy, or 
compact mass, which cements these pieces, as it were, into a whole. To 
assist in furnishing a clearer view of the subject, let us illustrate the manner 
in which some of such species of rock may arise. In attentively examining 
the action of currents of water on masses of rock, we find that fragments 
of these are, by various agencies, broken off or loosened, and carried 
away. In the transportation the sharp corners and edges are worn down 
by the attrition produced between the different pieces, until finally the mass 
is reduced to an ellipsoidal or globular form. In this way may be produced 
boulders, pebbles, and sand. The size may vary from that of coarse sand 
to blocks or masses of considerable magnitude, depending upon the original 
size of the fragment, the hardness of the material, and the length of time 
during which the rolling has continued, as also upon the velocity of the 
current. Stones may in this way be brought from the heads of streams, and 
carried out into gulfs of the sea, there to be distributed in layers. The fine 
sand or comminuted matter suspended in the water, whether resulting from 
this attrition or from other causes, will be deposited when the current is 
weakened by its expansion into the aforesaid gulf or bay, and will occupy 
the interstices of these rounded stones. By the upheaving of the bottom, 
exposure to the atmosphere, or igneous action, the mass is indurated in the 
course of time, and thus a truly heteronomic rock is exhibited. This 
method of formation does not apply to all heteronomic rocks, many of them 
being produced by the destruction of isonomic rocks in other ways. An 
essential difference in character enabies us to distinguish heteronomic rocks 
into conglutinates and congregates. 
a. Conglutinates. 
In conglutinates the connexion of the particles or parts is effected by a 
combining medium of different character. This difference in character 
may be only in the state of aggregation, since the parts may be cemented 
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