DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 655 
shales only. For field use in connection with arkose, graywacke, 
conglomerate, breccia, and grit, other sizings for fine-grained, 
medium-grained, coarse-grained are advocated. 
Fine-grained arkose (graywacke, etc.)........... grains to % in. in diameter 
Medium-grained arkose (graywacke, etc.)....grains from % to } in. in diameter 
Coarse-grained arkose (graywacke, etc.)........ grains over % in. in diameter 
In most cases, particularly in conglomerates, maximum and mini- 
mum as well as average sizes should be noted. 
Some term seems needed to denote a rock which is mainly an 
even-sized matrix, but contains a few pebbles over { in. in diameter. 
Pebbled sandstone, pebbled limestone, pebbled shale are advanced. 
3. Degree of cementation.—The microscope permits the abandon- 
ment of the field terms to denote hardness: soft, subsoft, hard, 
superhard. The following incomplete table is tentatively offered 
for criticism. 
Rock Not Well Cemented Cemented; Grains Not Cemented; Grains 
(Primarily field terms) Interlocked Interlocked 
Sandrock Sandstone to quartzite ee 
sandstone Paraquartzite 
Limerock Limestone [eee 
Magnesian limerock Dolomite eens 
Clay Shale Slate 
Arkose Arkosite to quartzite Arkositite 
arkose 
Glauconite sandrock Glauconitite (existent ?) Existent ? 
Ferrite sandrock Ferrite Existent ? 
Gravel Conglomerate Quartzite- 
conglomerate 
Most of these terms are comprehensible at a glance. By ortho- 
quartzite is meant rock cemented only through infiltration and 
pressure. By paraquartzite is meant quartzite mainly originating 
through contact metamorphism. 
Doubtless it is illogical to remove quartzite, marble, and slate 
from the category of metamorphic rocks. However, quartzites, 
slates, and marbles are universally given a place in vertical sections 
and in geologic folios are described under sediments. Schists and 
gneisses are not so treated. Practical exigencies would seem to 
override Aristotelian ‘“‘laws.” 
