WEATHEEING OF ULTRA BASIC EOCKS 209 



alkalies and rich, in iron or aluminum and magnesian com- 

 pounds. Owing, further, to their poverty in alkali-bearing 

 silicates, the process of decomposition must be less complex, 

 consisting essentially in hydration, oxidation, and a produc- 

 tion of iron, lime, and magnesian carbonates and a liberation 

 of chaleedonic silica. 



During the process these rocks as a rule become brownish, 

 and, on the surface, often irregularly checked with a fine net- 

 work of rifts which become filled with secondary calcite, mag- 

 nesite, and chalcedony. 



The deep green serpentines of Harford County, Maryland, 

 weather slowly down into a gray-brown soil, which consists of 

 60.17% silica, 10.40% of the iron oxides, 14.81% of alumina, and 

 only 7.23% magnesia. The fresh rock, on the other hand, car- 

 ries nearly 40% of magnesia, 8.50% iron and other metallic 

 oxides, and less than one-half of one per cent of alumina. 



Natural joint blocks occur in which the preliminary stages 

 of weathering are manifested by a brown, ferruginous, though 

 tough and hard, vesicular crust of from a millimetre to two or 

 more centimetres' thickness, enclosing the slightly hydrated but 

 otherwise unchanged material. 



An interesting case occurring near Manheim, in Herkimer 

 County, New York, of decomposition among igneous rocks of a 

 very basic nature and containing serpentine as a result of al- 

 teration from olivine has been described by Professor C. H. 

 Smyth, Jr.,^ and may be well referred to in detail. The original 

 rock consisted essentially of olivine, biotite, and probably melilite, 

 with accessory magnetite, apatite, and perofskite. Through 

 alteration the olivine had gone over into serpentine, as above 

 noted, and even in the freshest samples obtainable there was 

 some secondary calcite. In its unweathered state the rock, which 

 occurs in a dike some 26 to 30 inches in width in Calciferous 

 sand rock, is compact, dark-gray to nearly black in color, with 

 only a dark-brown mica conspicuous to the unaided eye. The 

 weathered portion is of a light yellowish-brown color and so 

 thoroughly disintegrated that the material can be easily scooped 

 out with the hand. 



The material selected for analysis was taken at a depth of four 



^Bull. Geol. Soc. of Am., Yol. IX, 1898, p. 257. 

 15 



