114 



AQUEOUS BOCKS 



have been rearranged in beds of varying thickness through the 

 mechanical agency of water. They are, in short, more or less 

 consolidated beds of sand and gravel. In composition and tex- 

 ture, they vary almost indefinitely. Many of them in which the 

 particles have suffered little during the process of disintegration 



and transportation, are 

 composed of essentially 

 the same materials as the 

 rocks from which they 

 were derived. Others 

 have had the softer and 

 more soluble minerals re- 

 moved, leaving the sand 

 composed mainly of the 

 hard, almost indestruc- 

 tible mineral quartz. 



In structure, the sand- 

 stones also vary greatly, 

 in some the grains being 

 rounded, while in others 

 they are sharply angular. 

 Figure 9 shows the mi- 

 croscopic structure of a brown Triassic sandstone from Portland, 

 Connecticut. 



The material by which the individual grains of a sandstone 

 are bound together is as a rule of a calcareous, ferruginous, or 

 siliceous nature; sometimes argillaceous. The substance has 

 been deposited between the granules by percolating water or 

 during the process of sedimentation, and forms a natural 

 cement. It sometimes 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 in this work classed with the crystalline rocks. 



Upon the character of this cementing material and the close- 

 ness with which the grains are bound together, is very largely 

 dependent the power of the stone to resist disintegration under 

 the trying action of percolating carbonated waters and the 

 mechanical action of heat and frost. The calcareous, and to a 

 less extent the ferruginous cements are liable to removal in 

 solution, allowing the rock to fall away to sand, or at least 



Pig. 9. — Mierostructure of sandstone, 

 Portland, Connecticut. 



