Classification of Sedimentary Rocks. — Grabau. 239 
scarcely be queslioned, for although the explosion which brings 
about the clastic condition of the lavas within, or the rocks 
surrounding the volcanic vent, occurs within the earth, it is ex- 
ternal with reference to the rock mass aflfected. 
While the rocks chiefly involved in the destruction due to 
the volcanic explosions are the lavas — fresh and old, and the 
previously formed tufit's, etc., composing the volcano, — the 
basement rocks may also be involved, thus producing non- 
igneous pyroclastics, which are, however, of relatively^ slight 
importance. Rocks shattered by earthquakes form a connect- 
ing link between the true pyroclastics and the autoclastics. The 
limiting states of pyroclastic activity are vaporization on the one 
hand, and fusion on the other, or the return of the matter to 
such states from which it may be redeposited in an endogenetic 
manner. 
The Aiitoclastic group of rocks comprises all rocks shat- 
tered or crushed within the earth either by pressure of one 
rock mass upon the other, or by movement of rocks over each 
other. Fault-breccias and the material of "crush zones" must 
be classed here, while earthquake shattered rocks may also be 
included here, though as before noted, they are transitional to 
the pyroclastics. By far the most important autoclastic pro- 
ducts, however, are those resulting from glacial erosion. Ice, 
and all the material frozen in it, is a part of the earth's crust 
while it exists, and hence any material ground up between the 
ice and the rock on which it moves, is of the type of the material 
crushed between other moving rock masses (fault breccia). 
Furthermore, since all ice-transported material has received its 
most characteristic features from that agent, we may with pro- 
priety include such material in this group, even though it was 
originally broken by atmospheric agencies (atmoclastic). In 
other words, I would consider all glacial material — exclusive 
of such as has been worked over by water — as belonging to 
the group of autoclastic rock material — and if consolidated 
forming autoclastic rocks, just as I would consider consolidated 
wind-blown material as aeolian or anemoclastic rock, whether or 
not the wind was chief factor in causing the fragmental con- 
dition. 
The Atmoclastic group typically comprises rocks broken up 
in situ, either by chemical or mechanical means, and recemented 
