INDUSTRIAL PROGRESS DURING THE YEAR 1876. xcvii 



INVERSIONS OF STRATA. 



Hunt further points out that we have in this region many ilhistra- 

 tions of what seems to be a common foot, that when newer strata 

 dej)osited around the base of a ridge or barrier of older rocks have 

 been subjected to folding, they often assume a dip toward the bar- 

 rier. The movement in such cases, apparently due to lateral press- 

 ure, has sometimes even involved the older rock itself, and caused it 

 to be folded over, so that the newer strata are actually inverted and 

 overlaid by the older and unconformable rocks, as may be seen in 

 the South Mountain in New Jersey, 



ORIGIN OF LIMONITES. 

 The presence in these Primal and Auroral strata of great beds and 

 masses of pyrites, and of others of carbonate of iron, is also noted ; 

 and where, as is often the case, these rocks are deeply altered by 

 meteoric agencies, the sulphuret and carbonate have been changed 

 into hydrated oxide of iron, which, in the form of limonite, abounds 

 in the clays resulting from this jDrocess of decomposition. 



DECAY OF ROCKS. 



This decay of the strata, so noticeable in these Lower Taconic 

 rocks, is also seen in the Laurentian gneisses of the South Mountain, 

 where these are protected from erosion. In such cases the par- 

 tially decomposed feldspathic rocks often contain nuclear masses 

 of the unchanged rock boulders of decomposition. Belt has 

 pointed out in Nicaragua examples of the slipping or sliding of 

 such decayed rocks on a mountain-side, by which undecayed por- 

 tions are carried down the slope and involved in the displaced 

 material. Kerr has lately further illustrated the question in North 

 Carolina by showing that alternate freezing and thawing of such de- 

 cayed rocks on a declivity may cause the descent of the material, 

 with scratching of the surface, thus giving rise locally to phenomena 

 similar to those often ascribed to sub-aerial glaciers. Such move- 

 ments of decayed crj-stalline rocks, either with or without the aid 

 of congelation, have no doubt in some regions been mistaken for the 

 result of ice-action. 



LAURENTIAN AND NORIAN OF THE ADIRONDACKS. 

 Professor James Hall has studied the arrangement and distribu- 

 tion of the crystalline rocks in parts of the Adirondack region, and 

 has verified the old observations of the Canadian Survey that the 

 labradorite and hypersthene rocks, with titanic iron ores the Norian 

 series rest unconformably upon the Laurentian gneisses, which con- 

 tain the pure magnetic ores of the region. 



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