BUILDING AND ORNAMENTAL STONES. 443 



M.— FRAGMENTAL ROCKS. 



(1) SANDSTONES, BRECCIAS, AND CONGLOMERATES. 



(a) Composition and Origin. 



Sandstones are composed of rounded and angular grains of sand so 

 cemented and compacted as to form a solid rock. The cementing ma- 

 terial may be either silica, carbonate of lime, an iron oxide, or clayey 

 matter. Upon the character of this cementing material, more perhaps 

 than upon the character of the grains themselves, is dependent the 

 color of the rock and its adaptability for architectural purposes. If 

 silica alone is present the rock is light colored and frequently so in- 

 tensely hard that it can be worked only with great difficulty. Such 

 are among the most durable of all rocks, but their light colors and 

 poor working qualities are something of a drawback to their extensive 

 use. The cutting of such stones often subjects the workmen to serious 

 inconvenience on account of the very fine and sharp dust or powder 

 made by the tools, and which is so light as to remain suspended for 

 some time in the air. The hard Potsdam sandstones of New York 

 State have been the subject of complaint on this score. If the cement 

 is composed largely of iron oxides the stone is red or brownish in color 

 and usually not too hard to work readily.* When the cementing ma- 

 terial is carbonate of lime the stone is light colored or gray, soft, and 

 easy to work. As a rule such stone do not weather so well as those 

 with either the siliceous or ferruginous cement, owing to the ready 

 solubility of the lime in the water of slightly acidulated rains; the 

 siliceous grains become loosened and the rock disintegrates. The 

 clayey cement is more objectionable than any yet mentioned, since it 

 readily absorbs water and renders the stone more liable to injury by 

 frost. Many sandstones contain little if any cement, but owe their 

 tenacity simply to the pressure to which they were subjected at the 

 time of their consolidation. Such stones are generally of a grayish hue, 

 easy to work, and if the amount of cohesion be sufficiently great, are 

 very durable. The finer varieties of these stones, such as the Euclid 

 " bluestone" and u Berea grits," are utilized in the manufacture of grind- 

 stones and whet stones. Since they contain little cementing material 

 they do not become polished when exposed to wear, but crumble slowly 

 away, presenting always fresh, sharp surfaces to be acted upon. In cer- 

 tain of our Potsdam sandstones the siliceous cement is found to have 

 so arranged itself with relation to the grains of sand as to practically 

 convert it into a crystalline rock or quartzite. This has already been 

 referred to in the chapter on microscopic structure. 



* Julien states that in the Tertiary sandstones of the Appalachian border the ferru- 

 ginous cement is largely turgite; in the Triassic and Carboniferous sandstones it is 

 largely limonite, aud in the Potsdam sandstones of Lake Champlain and the southern 

 shore of Lake Superior it is largely hematite. (Proc. A. A. A. S., Vol. xxvm, 1879, 

 p. 408.) 



