286 Tlie American Geologist. May, i899 
by heat and pressure in the presence of water minerals are 
formed. Perhaps fair illustrations could be transcribed from 
experimental synthesis and mechanical observations, as W. 
Spring's determination of conchoidal fracture and crystalline 
texture of dry pulverized chalk, after seventeen years in a screw 
press under very high pressure, while extracts could be made 
from Becker's account (now in some particulars doubtful) of 
the process of metamorphism observed by him in the meta- 
morphic rocks of California. (U. S. Geol. Surv. Monograph 
No. XIII.) 
After the idea has been dwelt upon, examples of possible 
sediments, muds, etc., and schists, slates, or marbles resulting 
from them by metamorphism can be introduced. 
Sedimentation, consolidation, chemical precipitation are 
more simple. A direct appeal to the senses can here be made. 
The cylindrical jar, with its layers of sand, gravel, clay, etc., 
in a series under the water, illustrates sedimentation, the gravel 
formed into a conglomerate by carbonate of lime or iron oxide 
cementing its pebbles (many morainal banks will show this 
process of rock making) illustrates consolidation, and the lime 
deposit from water as the linings of "hard cake" in boilers, the 
shell formed organically in water, and the growing stalactite 
and stalagmite, illustrate chemical precipitation. 
V. Classification. 
Broadly, rocks can be grouped for popular instruction im- 
der fragmental and crystalline, and crystalline under igneous 
and metamorphic, and the series arranged in the order of frag- 
mental, metamorphic, igneous. Under fragmental breccias, 
conglomerates, sandstones, limestones, slate would be exhibit- 
ed. Under metamorphic schists and marble, and under the ig- 
neous, the acid to basic series as devised and recommended b}' 
Van Hise, Weed, Turner, Cross and Diller, and admirably 
summarized by Merrill in "Rocks, Rock-Weathering and 
Soils." 
V I . Distrihition . 
Under distribution maps showing the areas occupied by 
sedimentary, metamorphic, and igneous rocks are recom- 
mended. An elaboration of this section can easily be devised 
by sets of rocks from different parts of the country or of the 
