EIvISHA MiTCHElvIv SCIENTIFIC SOCIETY. 35 



have been collected almost at the surface in which 

 scarcel}^ a trace of alteration could be seen. It is ^Yell 

 ^^Inown that olivine is remarkabh' prone to alteration, 

 either to serpentine, throug-h hydration, or to iron ox- 

 ides and soluble carbonates, in ordinary surface weath- 

 ering-. Even if it might be conceived that a pre-ex- 

 isting- olivine lava had been beaten down by the waves 

 and deposited as beds of sand along- the beach, as we are 

 told act nail V occurs on ^ome of the Haw^aiian coasts, it 

 would be extremely difficult to imag-ine this rock, with 

 all its contained water, placed under conditions nec- 

 essary to complete solidification into a sandstone, and 

 carried throug-h all the metamorphosing- conditions 

 that the enclosing- gneisses have certain!}- underg-one, 

 and at last, by erosion to the very heart of the Appa- 

 lachians, broug-ht to the surface unaltered. With a 

 sandstone of almost pure silica, I can imagine such an 

 evolution possible; but with unstable olivine, the hy- 

 pothesis seems entire!}^ untenable. 



The rapid rate of erosion in the mountain region and 

 the ease with which the granular olivine rock crum- 

 bles down under surface weathering, may well account 

 for the freshness of the present exposures. But beds 

 of olivine sand are not formed under these conditions, 

 even in the channels of neighboring streams. Hence, 

 I am led to the conclusion that no theory of sediment- 

 ary origin can adequately account for existing condi- 

 tions, and that these olivine rocks are now practically 

 in the state in which they originally solidified from the 

 molten magma. 



SUMMARY OF EVIDENCE. 



I have endeavored to show that the peridotites of the 

 South Appalachian region must be regarded as plu- 

 tonic ig-neous rocks for the following reasons: 



