246 , The American Geologist. April, 1904. 
not referable to either dynamic or thermic metamorphism. As 
examples of such further diagenic action may be cited the de- 
position of secondary silica around sand grains, and the crys- 
tallization of such silica in optical continuity with the original 
grain ; the crystallization of clastic limestones, and others of 
this type. The products of sethobalism and symphrattism, i.e. 
the sethobalic and symphrattic elastics need not be further con- 
sidered here. Their proper position in the system is after the 
diagenic members of the same group. 
Note on the use of the Table of Bxogenetic rocks. 
Any clastic rock may generally be at once placed under one 
of the three textural groups rudyte, arenyte, or lutyte, which 
grouping is best considered first purely for convenience sake. 
Sometimes nothing more can be determined about a given 
rock, without special chemical or microscopic examination. 
When the composition is determined, a further step is made in 
the classification, but the primary group can generally not yet 
be ascertained. If the rock is of simple composition, and pure, 
it may be classed as follows : If pure silica, as a silicirudyte, 
silicarenyte, or silicilutyte ; or if impure, as a calcareous, argil- 
laceous, feldspatic, glauconitic, micaceous, etc., silicirudyte, sili- 
carenyte, or silicilutyte. If pure lime, it is a calcirudyte, cal- 
carenyte or calcilutyte; or if impure, a siliceous, argillaceous, 
magnesian, ferruginous, carbonaceous, etc., calcirudyte, calcar- 
enyte or calcilutyte. Finally if pure clay (kaolin), it is an ar- 
gil lutyte, though most commonly the rock would prove a sili- 
ceous or calcareous or carbonaceous argillutyte. If the rock is 
complex in composition, with neither free quartz, lime nor clay 
predominating to give it a distinctive character, it may still 
be a siliceous (silicate) rudyte, arenyte or lutyte, as the case 
may be, or it may even be an argillaceous or calcareous rudyte, 
or arenyte, where those minerals are present in minor quantity. 
So far the determinations are generally possible from hand 
specimens. But not until its primary relations, i.e. its geologic 
or field relations, are determined, can the rock be placed in its 
primary group. Exceptions to this are fossiliferpus rocks, and 
rocks belonging to the pyroclastic or bioclastic divisions, which 
can generally be recognized in hand specimens. The pyroclas- 
tics and bioclastics may at once be classed as pyrorudytes, (vol- 
canic breccias, etc.) pyrarenyte, or pyrolutytes (volcanic tufFs^ 
