422 DESCRIPTIONS OF ROCKS. 



Cherbourg and the syenyte of the Yosges bore 700 to 1,000 

 kilograms"; and other coarse granites, in which the large 

 crystals of feldspar were in part decomposed, bore only 100 

 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 lbs. a square inch (about 6 c. 

 m. sq.) : granite of Westerly, E. L, 17,750; id. of Rich- 

 mond, Va., 21.250 : syenyte of Quincv, 17,750 : marble of 

 Tuekahoe,'X. Y.. 12.950 ; id. of Dorset, Yt.. 7,612 : lime- 

 stone of Joliet, 111.. 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 : id. of Medina, X. Y., 

 17,250 ; id. of Dorchester, X. 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 Yitry and other localities, mostly one-third to 

 one-half. Tournaire and Michelot found, for the chalk of 

 the Parts basin, the pressure required when wet two-ninths 

 of that required when the rock had been dried at a tempera- 

 ture considerably above 212 z 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 crystalline rock be coarse or tine 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 

 sliees 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 inacroscopicaUy, 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 polari scope. 



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. 



