850 



ROCKS. 



mulation. But others, or even certain parts of beds that 

 elsewhere contain these indications, have a crystalline tex- 

 ture. A limestone bed may be compact in one part, and 

 granular or crystalline, like statuary marble, in another. 

 Here is an effect of heat on a portion of the bed ; heat, 

 which has acted since the rock was deposited. Other rocks, 

 such as mica slate, gneiss, and probably some granites, have 

 thus been crystallized. They are called metamorphic rocks. 



In these few general remarks on the structure of the globe, 

 ' we have distinguished the following general facts : 



1. The great variety of alternations of sandstone, conglo- 

 merates, clay shale, and limestones. 



2. The existence of igneous rocks in beds and intersect- 

 ing dikes or veins. 



3. The mechanical structure of sandstone, conglomerate, 

 and shales. 



4. The crystalline character of igneous rocks. 



5. The crystalline character of many stratified or sedi- 

 mentary rocks, arising from the action of heat upon the beds 

 of rock themselves, after they were first formed. 



We follow this comprehensive survey of the arrangement 

 and general nature of rocks, with descriptions of the more 

 prominent varieties and a mention of their applications in 

 the arts.* 



* One of the most important uses of stone is for architectural pur- 

 poses. The character of the material depends not only upon its dura- 

 bility, but also its contraction or expansion from changes of tempera- 

 ture. This latter cause occasions fractures or the opening of seams, 

 and produces in cold climates serious injuries to structures. The fol- 

 lowing table, by Mr. A. J. Adie, gives the rate of expansion in length 

 for different materials, for a change of temperature of 180° F. — Proc. 

 Roy. Soc. Edinb., i, 95, 1835. 



Granite, -0008968— -0007894 



Sicilian white marble, -00110411 



Carrara marble, -0006539 



Black marble, from Galway, Ireland, -00044519 



Sandstone, (Craigleith quarry, Scotland; -0011743 



Slate, Penryhn quarry, Wales, -0010376 



Greenstone, -0008089 



Best brick, -0005502 



Fire brick, -0004928 



Cast iron, -00114676— -001102166 



Rod of wedgewood ware, -00045294 



