92 THE MICROSCOPIST. 



red sandstone, magnified seventy-five diameters. Exter- 

 nally it resembles dirty green bottle-glass, but shows in 

 the microscope an arborescent crystallization of a green 

 pyroxenic mineral in a colorless felspar base. Plate II, 

 Fig. 48, shows auriferous diorite from Chili, consisting of 

 felspar, with hornblende and crystals of iron pyrites, mag- 

 nified thirty diameters. Plate II, Fig. 49, is a section of 

 granite from Cornwall, with crystals of orthoclase, hexag- 

 onal crystals of brown mica, and colorless quartz, which 

 a higher power shows to contain fluid cavities, magnified 

 twenty-five diameters. Plate II, Fig. 50, a volcanic rock 

 from Peru, composed of felspar, dark crystals of augite, 

 hexagonal crystals of dark mica, and a little magnetic 

 oxide of iron, magnified six diameters. Plate II, Fig. 

 5 1 , lower silurian roofing-slate, cut at right angles to the 

 cleavage, showing that the latter is not due to crystalline 

 but to mechanical arrangement, magnified two hundred 



O O 



diameters. Plate II, Fig. 52, is an oolitic specimen from 

 Peru, regarded as an eruptive rock by D'Orbigny, but 

 shown in the microscope to be a mere aggregation of 

 sand, etc., without the crystalline character of eruptive 

 rocks. 



Materials of Organic Origin. Rocks and strata derived 

 from plants or animals may be arranged in four groups : 

 1. The calcareous, or those of which limestones have been 

 formed, as corals, corallines, shells, crinoids, etc. 2. The 

 siliceous, which have contributed to the silica, and may 

 have originated flints, as the microscopic shields of dia- 

 toms and siliceous spiculae of sponges. 3. The phosphatic, 

 as bones, excrement, etc. Fossil excrements are called 

 coprolites, and those of birds in large accumulations, 

 guano. 4. The carbonaceous, or those which have afforded 

 coal and resin, as plants. 



To examine the structure of coal, it is necessary to have 

 very thin sections. From its friability, this is a process 

 of great difficulty. The Micrographic Dictionary recom- 



