654 A. J. TIESE 
In pursuance of this belief, the renaming of rocks falling under 
A 1 and B 2 is considered from five points of view: (1) rock source 
of the sediment; (2) size and shape of grain; (3) degree of cementa- 
tion; (4) mineral composition; (5) fossil content. 
1. Rock source.—Seemingly, there is much satisfaction in speak- 
ing of aqueous and eolian sediments, perhaps even more in men- 
tioning an anemopotamoclast. Nevertheless, such terms do not 
aid the sedimentary-rock student in the same way that peridotite 
or bostonite aids the igneous-rock student. True, one cannot. 
assert that sands from gabbro rock sources will look different from 
sands from granite rock sources; of the requisite thin sections there 
are too few descriptions to tell. In the writer’s own experience, 
nevertheless, Cambrian sands derived from the microcline granites 
of Wyoming do seem to have a characteristic appearance under the 
microscope. Would it not secure definiteness of description if 
sedimentary rocks, mainly, of course, sandstones, had prefixed to 
their colorless names such terms as granitogene, gabbrogene, 
quartzitogene ? 
2. Size and shape of grain.—Shape of grain should be an essen- 
tial part of a microscopic description. The terms angular, near- 
angular, subrounded, rounded might constitute a useful series. 
Size of grain is, under the microscope, capable of exact delimi- 
tation. The scale used in connection with igneous rocks is not, 
however, subdivided enough. Accordingly, the adoption of a 
modification of the New York City Aqueduct standard is advocated. 
The terms suggested chiefly, perhaps, apply to sandstones, some- 
what to limestones, little to shales. 
Sedimentary rock very coarse-grained...................- grains over I mm. 
Sedimentary rock coarse-grained.............. grains between o.5 and 1 mm. 
Sedimentary rock medium-grained.......... grains between 0.25 and 0.5 mm. 
Sedimentary rock fine-grained.............-. grains between o.1 and 0.25 mm. 
Sedimentary rock very fine-grained........ grains between 0.05 and o.1 mm. 
Sedimentary rock superfine-grained.................. grains below 0.05 mm. 
By this scale, of course, arkoses, graywackes, conglomerates, 
breccias, even most grits would be very coarse-grained. It will be 
observed, however, that the terms in the table refer to measure- 
ments under the microscope and for sandstones, limestones, and 
