Classification of Sedimentary Rocks. — Graban. 235 
of guano, but these frequently contain so high a propor- 
tion of alkalies, that they may be considered transitional be- 
tween the first and second groups of the present table. 
Siliceous organic deposits (c) in the unstratified or mas- 
sive condition, formed chiefly through the agency of plants, 
occur in both fresh and salt water. But by far the largest de- 
posits of organic silica are in the form of stratified oozes^ such 
as radiolarian ooze, diatomaceous deposits and those composed 
of sponge spicules and other minute siliceous organisms. Flints 
and cherts are derived from these by a process of solution and 
redeposition in purely hydrogenic manner. Ferruginous or- 
ganic deposits (d) comprise iron deposits due to the physiolog- 
ical activities of bacteria resulting in organic limonites, while 
carbonaceous rocks (e) must be considered in the light of our 
present knowledge as wholly of biogenic origin. Peat, lignyte, 
and bituminous coal are the typical carbonaceous biogenic rocks, 
the latter belonging in a measure to the alteration or diagenic 
.products. Secondary derivatives through diagenic alteration 
are found in the hydrocarbons, petroleum, asphalt, etc., and fur- 
ther metamorphic products in some anthracite coals and in 
native coke. 
As has already been indicated, a rock of any of the above 
groups does not generally remain for an indefinite period with- 
out becoming more or less altered, through the influence of the 
environment. Extreme alteration through the influence of heat, 
with or without accompanying pressure, is familiarly known as 
metamorphism. But there are a great many alterations of rocks 
which would not come under this head, unless the meaning of 
the term is extended to its limit, which,. though desirable, is 
not always done. Walther has adapted the term diagenesis 
from V. Guembel, who used it in a somewhat different sense,* 
and applied it to all those physical and chemical changes zvhich 
a rock mass undergoes after its deposition, aside from those 
caused by pressure during orogenic disturbances and bv igne- 
ous activity. Deep seated alteration of igneous rocks, dehvdra- 
tion of gypsum, crystalization of organic limestones, and the va- 
rious changes which vegetable deposits undergo, may serve as 
examples of diagenesis in the class of endogenetic rocks. 
• Wai.turr, Johannes. Einlehung in die Geotogie, s. 693. 
