282 The A?nerica?i Geologist. May, i899 
demonstrations the essential elements should be sharply indi- 
cated, and the provisional or accidental or sporadic nature of 
the accessories also clearly stated, the whole illustrated by some 
homely analogy, as cement being typically a mixture of sand 
and lime in which brick dust, or clay, or glass, or pebbles, 
might be mingled. 
The Idea of Rock secondly, as not necessarily a compound 
of minerals, but an extension in mass and area of a single 
mineral, can be expressed in contrasted specimens of cal- 
cite crystals and dolomite crystals with blocks of marble and 
limestone, gypsum in crystals with large fragments of alabaster 
or rock plaster, serpefitine hand specimens with serpentine 
masses, the whole enforced by printed explanations and pho- 
tographs of marble and gypsum quarries. 
The Idea of Rock, thirdly, can be expressed by tables of 
physical constants, and by statistics of industry in the use of 
rock. Tables of physical constants are too numerous to de- 
mand specification. A sheet of them well printed could help- 
fully be placed in a section of the cases of exhibition. Exam- 
ples chosen from Merrill or the ninth census, may be here brief- 
ly quoted. 
Locality. Strength Wt ^^^^. 
■' persq.in. per eft. ^ 
Biotite Granite — Niantic, Conn 9,550 lbs. 162.5 lbs. 2.6 
Hornblende Granite — E. St. Cloud, Minn. 28,000 lbs. 163. i lbs. 2.609 
Gneiss — Madison Ave., N. Y 11, 250 lbs. 182.5 lbs. 2.92 
Trap — Taylor's Falls, Minn 26,250 lbs. 187.5 lbs. 3-00 
Limestone — Bardstown, Ky 16,250 lbs. 166.9 lbs. 2.67 
Marble — Lee, Mass 22,860 lbs. 
Marble — Stillwater, Minn 25,000 lbs. 172.6 lbs. 2.76 
Sandstone, Jordan, Minn 3,750 lbs. ii3.ilbs. 1.82 
Sandstone, Near Fort Snelling, Minn 20,000 lbs. 138.8 lbs. 2.22 
The contrasted strength or co-efficient of resistance to pres- 
sure between rocks in their bedding and on edge could be in- 
troduced in the preceding tables. Then might follow a sheet 
of chemical compositions. That, by photographs of buildings, 
with description of the stone employed in their construction, 
making the selection as broad as possible, including volcanic 
as w^ell as sedimentary rocks. Then some statistics of industry 
in stones taken from census reports or mineral resources of the 
United States. By such manifold data the dawning concep- 
tion of rock grows outward into a complete compass of its vari- 
ous aspects. 
