408 REPORT ON NATIONAL MUSEUM, 1886. 



rocks, but occur frequently in eruptive masses or bosses, invading rocks 

 of all ages up to late Mesozoic or Tertiary times.* 



They are very abundant throughout the Eastern and Northern United 

 States and the Rocky Mountain region. 



The average specific gravity of granite is 2.66, which is equal to a weight 

 of 166 J pounds per cubic foot, or practically 2 tons per cubic yard. Ac- 

 cording to Professor Anstedf granites ordinarily contain about 0.8 per 

 cent, of water, and are capable of absorbing some 0.2 per cent. more. 

 In other words, a cubic yard would in its ordinary state contain 3.5 gal- 

 lons of water. The crushing strength of granite is quitu variable, but 

 usually lies between 15,000 and 20,000 pounds per square inch, as will 

 be seen by reference to the tables. The average chemical composition 

 is as follows : 



Por cent. 



Silica 72.00 



Alumina 15. 07 



Iron peroxide 2.22 



Magnesia 5.00 



Lime 2. 00 



Potash 4.12 



So<la 2.9 



Loss by ignition 1.19 



(2) VARIETIES OF GRANITE. 



In classifying granites the varietal distinction is based upon the pre- 

 vailing accessory minerals. The more common varieties are muscovitc 

 granite, biotite granite, muscovite-biotite granite, hornblende granite 

 and hornblende-biotite granite; more rarely occur augite, epidote, 

 tourmaline, cordierite, and chlorite granites. The variety without any 

 accessory minerals is sometimes called granitell. Protogine is the name 

 given to granites like those of Mount Blanc, which have talc or chlorite 

 as the characterizing accessory. Pegmatite or graphic granite is a vein 

 rock containing scarcely any mica, but consisting almost altogether of 

 quartz and orthoclase. It owes its peculiar structure to the crystalliza- 

 tion of these two ingredients in long parallel and imperfect prisms so 

 that a cross-section shows peculiar triangular and polygonal figures 

 comparable to the letters of the ancient Greek or Phoenician alphabets. 



By far the larger proportion of the granites at present quarried in 

 the United States have mica, either muscovite or biotite, as the charac- 

 terizing accessory, and hence can be spoken of as mica granites. The 

 amount of mica present is of considerable economic importance. It 

 does not polish as easily as do quartz and feldspar, owing to its softness, 



* Professor Whitney considers the eruptive granites of the Sierra Nevada to be 

 Jurassic. Zirkol divides the granites described in the reports of the 40th parallel 

 survey into three groups : (1) Those of Jurassic age; (2) those of Paleozoic age; and 

 (\\) those of Archaean age. The granites of the Eastern United States, on the other 

 hand, are considered by geologists almost without exception as Arclnean. 



t Hull, Building and Ornamental Stones, p. 30. 



