16 THE MINEEALS CONSTITUTING BOOKS 



basic eruptive rocks. As a rule they are recognizable only as 

 feldspars by the unaided eye, and recourse must be had to the 

 microscope or to chemical tests for their final determination. 

 Examined in thin sections and by polarized light, they show a 

 beautiful parallel banding in light and dark colors, which is 

 due to multiple twinning, the alternate bands becoming light 

 and dark in turn as the stage of the microscope is revolved. 

 When the crystals are of sufficient size, this twinning is some- 

 times evident in the form of fine straight, parallel bands, or striie, 

 but in rock masses, as already noted, recourse must be made to 

 microscopic methods. In form the plagioclase of effusive rocks 

 is most frequently slender and elongated, lath-shaped^ as com- 

 monly described, and often with very perfect crystal outlines. 

 In the norites and gabbros, they are short and stout, imparting 

 a granular character to the rock. They occur frequently in 

 crystals of two or more generations, of which the earlier formed 

 are usually the largest and best developed. The common forms 

 are described in detail below: 



(1) Albite, or soda feldspar, occurs as an original constituent 

 in many granites in company with orthoclase; it is also found 

 in gneiss, the crystalline schists, and not infrequently in diorite, 

 phonolite, trachyte, and other eruptives. (2) Oligodase, soda- 

 lime feldspar, occurs like albite in the acid eruptives like gran- 

 ite and quartz porphyry, but is also a common constituent of 

 diorite, and the younger eruptives such as trachyte, the ande- 

 sites, and more rarely of the diabases. It is also a constituent 

 of many gneisses. (3) Labradorite, or lime-soda feldspar, is a 

 prominent constituent of the basic eruptives of all geological 

 ages, such as the norites, diabases, and basalts. Andesine and 

 bytownite are closely allied varieties of similar habit, the first 

 being a trifle more acid, and the second more basic Ihan labra- 

 dorite. (4) Anorthite, or lime feldspar, is also a prominent and 

 important constituent of the basic eruptives, and has been found 

 in meteorites and terrestrial peridotites. 



On account of their abundance and wide distribution, as well 

 as on account of the character of their decomposition products, 

 the feldspars are to be considered the most important of rock 

 constituents. As it is from the debris of the older feldspathie 

 rocks that have been made up a large proportion of all the 

 sedimentaries of more recent date, so too it may be claimed 

 that from the decomposition of this feldspathie constituent has 



