GEOLOGY. 225 



A very large proportion of the gold, however, is obtained in small 

 scales, by washing the earth which is dug up on the beds of the streams 

 or near their margin. A mass of the crude earth, as taken at random 

 from a placer, was tested by the Director of the United States Mint at 

 Philadelphia, and found to contain 264^ grs. of gold, being in value a 

 fraction over $10 to lOOlbs. It cannot, however, be reasonably sup- 

 posed that the average alluvial earth in the placers is so highly au- 

 riferous. 



COMPARATIVE STRUCTURAL FEATURES OF THE APPALACHIAN 



MOUNTAINS AND THE ALPS. 



AT the meeting of the American Association, Professor H. D. Rogers 

 presented an important communication on the " Structural Features of 

 the Appalachian Mountains, compared with those of the Alps and other 

 disturbed Districts of Europe." The characteristic features of the Ap- 

 palachians are, that on their southeast slopes the strata are invariably 

 doubled into oblique flexures or folds. Farther towards the northwest, 

 or central belts of the chain, these flexures are less perceptible, but the 

 inverted or northwestern side of the anticlinal curves dip much more 

 steeply than the southeastern. Advancing still farther across the chain, 

 these great flexures or arches of the rocks progressively expand, the 

 curvature of the northwestern slopes still, however, dipping very steep- 

 ly, while in the broad plateaus of the Alleghany and Cumberland Moun- 

 tains the arches or waves subside and dilate into symmetrical undulations 

 of equal and gentle curvature. Along all the southeastern border of 

 the chain, the prevailing dip is therefore toward the belt of active igneous 

 movement, where alone the strata are perforated by intrusive volcanic 

 rocks. These arches or waves are of great length, and, whether straight 

 or curved, exhibit a singular degree of parallelism and uniformity in 

 their style of flexure. In the southeastern and middle zones of the 

 chain, many of these great arches terminate in enormous longitudinal 

 faults or fractures, which are nothing else than inverted flexures broken 

 at some point in the inverted part of the anticlinal, producing the appa- 

 rent anomaly of an overlapping of newer strata by others of far older 

 date. Some of these fractures thus ingulf a thickness of nearly two 

 miles. The cleavage planes of the rocks are nearly parallel with the 

 average dip of the planes which symmetrically cut or bisect the anticlinal 

 and synclinal curves; and this law of position of the cleavage planes is 

 found to prevail equally in the plicated districts of the Rhine and the 

 Alps. Precisely analogous features to those which have been observed 

 in the Appalachians have been proved to belong to the paleozoic region 

 of the Ardennes, and the coal-fields of Belgium. In the more disturbed 

 tracts the strata are closely and sharply folded into almost absolute 

 parallelism, while farther north, in the carboniferous basins of the 

 Meuse, these flexures dilate precisely as in the sections of the Alle- 

 ghanies. The cleavage planes of the more contorted belt are, as in 

 the Appalachian region, planes which divide the curves, parallel to 

 the average dip of the axes. In the Jura, the same beautiful law of a 



