422 DESCRIPTIONS OF ROCKS. 
Cherbourg and the syenyte of the Vosges bore 700 to 1,000 
kilograms ; and other coarse granites, in which the large 
crystals of feldspar were in part decomposed, bore only 400 
to 600 kilograms. The green porphyry of Ternuay (Haute 
Sadne), bore 1,360 kilograms ; the basalt of Estelle (Puy 
de Dome), 1,880 kilograms. 
In trials by Gen. Gilmore, trap of New Jersey required 
to crush it 20,750 to 24,040 Ibs. a square inch (about 6 ec. 
m. sq.) ; granite of Westerly, R. 1., 17,750; id. of Rich- 
mond, Va., 21,250; syenyte of Quincy, 17,750 ; marble of 
Tuckahoe, N. Y., 12.950; id. of Dorset, Vt., 7,612 ; lime- 
stone of Joliet, Ill., 11,250; sandstone of Belleville, N. J., 
10,250; id. of Portland, Ct., 6,950; id. of Berea, O:, 
8,300; id. of Amherst; O., 6,650; 1d. of Medina iN ie 
17,250 ; id. of Dorchester, N. B., 9,150. 
When absorbent rocks are thoroughly wet the weight re- 
quired to crush them is greatly reduced. To crush wet chalk, 
according to trials by Delesse, required only one-third what 
it did when stove-dried; and for the limestone, ‘‘ calcaire 
grossier,” of Vitry and other localities, mostly one-third to 
one-half. Tournaire and Michelot found, for the chalk of 
the Paris basin, the pressure required when wet two-ninths 
of that required when the rock had been dried at a tempera- 
ture considerably above 212° F. 
Use of the Microscope in the Study of Rocks. The study of 
thin, transparent slices of rocks by the microscope is of in- 
terest whether the crystallie rock be coarse or fine in tex- 
ture; but it is particularly important when of the latter 
kind. There is no rock so opaque that it cannot be made 
transparent, or at least translucent, in thin slices. Such 
slices are examined by means of a polariscope-microscope. 
The increased use of the microscope in the investigation of 
rocks has led to the introduction, by way of distinction in 
methods of study, of the word macroscopic. An investiga- 
tion may be carried on macroscopically, that is, without the 
use of a microscope, excepting a pocket lens ; or microscopi- 
cally, that is, by the study of thin slices through the aid of 
the microscope and polariscope. 
The more important points ascertained by microscopic 
methods, as regards the mineral constitution of a rock, are 
the following: 
1. The presence or not of quartz; of a feldspar; of a 
chlorite. 
