AMERICAN GEOLOGY EATONIAN ERA, 1820-1829. 279 



Or, again, in writing on the weathering of rocks: 



The decomposition of granite is slow and when decomposed the unfriendly siliceous 

 grains are .easily washed away. There is neither vegetable nor animal matter in the 

 compound; it does not absorb moisture, letting the moisture percolate, nor does it 



retain heat. The soil made from gneiss is not washed away so easily, and the mica 

 yields more argillaceous matter. * * Micaceous schist and argillite decompose 



more rapidly and form a better, though not a good, soil. * The rivers of 



primary districts have rocky beds and precipitous hanks, etc. 



Van Rensselaer was one of the first to recognize the necessity of 

 exercising cure in the selection of stones for building purposes: 



We take our materials because they are near at hand, because they are cheap, and 

 hecar.se others have taken the same, in preference to searching out others which 

 * * * are more durable. Scarcely any one building in Europe or America of 

 modern construction at the end of one thousand years will have one stone left upon 

 another stone to denote the place where it stood, and the most splendid works of 

 modern architecture are even now hastening to decay from want of attention to this 

 subject. 



His remarks on anthracite coal offer an interesting illustration of a 

 disposition, still too frequently met with, to make the observed facts 

 conform to preconceived opinions. Anthracite or native mineral car- 

 bon, or "" blind coal." he wrote, is not regarded, strictly speaking, as a 

 coal, though combustible. 



It occurs in the primary rocks, and hence it is obvious that it did not proceed from 

 the decomposition of vegetable substances, since it is generally acknowledged that 

 the primary rocks were formed previous to the existence of vegetation. 



An analytical key or a synopsis, given with a view of facilitating 

 the study of geology, is of particular interest, since the methods of 

 modern petrography have shown how absolutely impossible it is to 

 classify rocks by a simple examination of the hand specimen and with- 

 out the aid of a microscope. It is difficult to imagine a student in 

 other than a thorough state of mental demoralization who should 

 attempt to identify and classify rocks from such a description, an 

 abstract of which is given below: 



SYNOPSIS OF THE OVERLYINU OR SUPERINCUMBENT ROCKS. 



FIRST DIVISION. 

 Simple, or apparently so. 



A. Wacke, of the German school. Resembles indurated clay, with an even and 

 smooth earthy, or an uneven, somewhat granular fracture, and a shining streak. 



a. Compact. 



b. Cellular; but generally in that case partly amygdaloidal and appertaining to 

 another division. 



B. Indurated clay, more or less hard, with an earthy and dull fracture. 

 a. Compact. 



This is different from the ferruginous clays found often with the trap rocks, which 

 pass into jasper. 

 h. Cellular. 



