536 



REPORT OF NATIONAL MUSEUM, 1890. 



rearranged in beds of varying thickness through the mechanical agency 

 of water. They are, in short, consolidated beds of sand and gravel. 

 In composition and texture they vary almost indefinitely. Many of 

 them having suffered little during the process of disintegration and 

 transportation, are composed of essentially the same materials as the 

 rocks from which they were derived. This is the case with the arkose 

 shown in specimens 39052 and 38135 from Rhode Island and France, 

 and the red Triassic sandstone shown in specimen No. 70067 from 

 Colorado. All of these were derived from granitic rocks and like them 

 consist of quartz, feldspar, and mica. Others, in which the fragmental 

 materials suffered more prior to their fiual consolidation, have had the 



softer and more soluble min- 

 erals removed, leaving the sand 

 composed mainly of the hard, 

 almost indestructible mineral 

 quartz. 



In structure the sandstones 

 also vary greatly, in some the 

 grains being rounded, while in 

 others they are sharply angular. 

 Fig. 92 shows the microscopic 

 structure of a brown Triassic 

 sandstone from Portland, Con- 

 necticut. 



The material by which the in- 

 dividual grains of a sandstone 

 are bound together is as a rule 

 of a calcareous, ferruginous or 

 siliceous nature ; sometimes ar- 

 gillaceous. The substance has 

 been deposited between the granules by percolating water and forms a 

 natural cement. It frequently happens that the siliceous cement is 

 deposited about the rounded grains of quartz in the form of a new 

 crystalline growth converting the stone into quartzite ; such are here 

 classed with the crystaliue rocks. (See p. 549.) 



The colors of sandstone are dependent upon a variety of circum- 

 stances. The red, brown, and yellowish colors are due to iron oxides 

 in the cementing constituent. In very light gray varieties the color is 

 that of the minerals themselves composing the stone. Some of the dark 

 colors are due to carbonaceous matter ; others to iron protoxide car- 

 bonates or clayey matter. (See color series.) 



Many varieties of sandstone are popularly recognized. Calcareous, 

 ferruginous, siliceous, or argillaceous sandstones are those in which the 

 cementing materials are of a calcareous, ferruginous, siliceous, or argil- 

 laceous nature. The name arkose is given to a coarse feldspathie sand- 

 stone derived frome granitic rocks. (Specimens No. 38135 from France 

 and 39052 from lihode Island.) 



Fig. 92. 



MiCRO-STRUCTURE OF SANDSTONE. 

 (Portland. Connecticut.) 



