238 The American Geologist. ^p^i- 1^04 
thermo-metamorphism, or aethohalism, giving rise to aetho- 
balic rocks, and 3. Dynamic or regional or pressure meta- 
morphism (Druck metamorphose) i.e. symphrattism, resulting 
in the production of symphrattic rocks. 
The Exogenetic or Clastic Rocks. 
In discussing next the clastic rocks, the agent active in the 
production of their oresent characteristics will be considered as 
of primary importance in making the larger divisions. It is 
not always possible to determine what was the cause of the 
clastic condition of a given rock, since other agents subse- 
quently active have produced those features which give the 
rock its most characteristic aspects. Thus a sand mass may 
owe its origin in part to atmospheric disintegration and in part 
to the mechanical activity of the water. Its final form, how- 
ever, may be given by aeolian action, the mass becoming finally 
by diagenism a consolidated sand-dune. It will, however, be 
observed that the agent last active, and therefore the one whose 
characteristic? were most strongly impressed upon the mass, 
i.e. the wind, is not only responsible for the form and structure 
which the deposit eventually takes on, but also, in part at least, 
modifies the original form of the component grains. Thus 
without overlooking the claims of the other agents, those of 
the wind may be considered as greatest and the rock is there- 
fore placed under the division of wind-formed elastics or ane- 
moclastics. 
The relative claims of the various agents active in the pro- 
duction of a given clastic rock once determined. — provided 
there is more than one agent, — it will be found that most rocks 
fall under one of five groups, though some rocks may fall so 
precisely half way between two groups, that it becomes a mat- 
ter of individual opinion where it should be placed. The five 
principal groups are : Pyroclastic, Autoclastic, Atmoclastic 
(including Anemoclastic), Hydroclastic and Bioclastic. (See 
diagram 2.) 
The Pyroclastic group is most typically represented by the 
tufis, volcanic breccias, etc. These may grade insensibly into 
the group of hydroclastics. From the nature of the agent ac- 
tive in their production it will be seen that they are most nearly 
related to the pyrogenic rocks in the endogenetic division. 
That they themselves belong to the exogenetic group, will 
