56 



LITHOLOGICAL GEOLOGY. 



closely that the two cannot often be distinguished except by chemical 

 means. Like calcite, it constitutes many limestone strata, both massive 



Fig. 42. 



and crystallized. When dropped in powder into dilute muriatic 

 acid, it effervesces very feebly, if at all, in the cold ; but, on heating 

 the acid, there is a brisk effervescence produced. The angles between 

 its cleavage faces is 106° 15', and this, with crystallized specimens, is an 

 important means of distinction. Composition, carbonate of lime 54*4, 

 carbonate of magnesia 45*6 = 100. 



Among sulphates, the only very common species is Gypsum. It is a 

 very soft mineral, one of the few that may be easily impressed with 

 the teeth, and without producing a grating sensation. It is often mas- 

 sive and very fine granular, and of various colors from white to black ; 

 the white is common alabaster. It also occurs in crystals and crystal- 

 line masses. Figures 44, 45 give two of the forms of the crystals. 



Fig. 44. 



Fig. 45. 



\ 



It cleaves in broad pearly plates or 

 folia, which look like mica, but are softer 

 and not elastic. Unlike limestone and 

 other minerals, a little heat reduces it to 

 powder, making the common plaster of paris of the shops. It consists 

 of sulphuric acid 46-51, lime 32-56, water 20-93 = 100. 



Sulphate of lime also occurs without water, and is then called anhy- 

 drite ; the crystallization is very different, cleavage affording rectangular 

 blocks or plates. 



Besides these very abundant rock-making species, there are the following of quite 

 common occurrence. 



Anhydrous Silicates. Nephelite, a colorless to grayish-green and greenish min- 

 eral (also of other shades), related somewhat to the feldspars, and having the place of a 

 feldspar in some igneous rocks. Its crystals are hexagonal prisms. Silica, alumina, 

 eoda and potash are the principal constituents. 



Leucite. A white or grayish-white mineral occurring in 24-sided crystals resembling 

 Fig. 47 ; it takes the place of a feldspar in igneous rocks, at Vesuvius and some other 

 European localities. Silica, alumina, and potash are its constituents. 



