SEDIMENTARY AND METAMORPHIC ROCKS 73 
The quartz will waste very slowly, and then only as 
it is dissolved, for it will not be altered chemically. 
The feldspar and the hornblende, being more complex, 
are less durable, and they soon commence to change, 
and finally become a clay from which some of the ele- 
ments go off in solution. Left without support, as a 
result of the decay of these minerals, the quartz grains 
fall out and the granite crumbles (Fig. 47). From the 
origmal minerals of the rock, three quite different 
products result: (1) soluble salts, (2) fine clayey frag- 
ments, and (3) larger grains of pure quartz. What is 
true of granite is true in greater or less degree of all 
rocks; and everywhere in Nature’s great laboratory 
such changes as these are in progress at the surface. 
According to the way in which these mineral products 
are gathered into layers or strata, we have three differ- 
ent groups of sedimentary rocks: (1) fragmental or 
clastic, (2) chemical precipitates, (8) organic. The 
second, and some members of the first and third, are 
aqueous; nearly all are stratified; and some of each 
group are truly sedimentary. 
Fragmental or Clastic Rocks. — Origin. These are 
composed of distinct fragments of other rocks, and 
these particles may be gathered into layers by one 
of four means: (1) the wind, (2) ice, (3) water, (4) vol- 
canic eruption. 
The wind can carry only fine grains of clay or sand; 
