158 G. P. MERRILL — WEATHERING OF MICACEOUS GNEISS. 



Columbia rocks, and thus no criteria are afforded for estimating the rela- 

 tive rapidity of the process. The amount of material lost through leach- 

 ing and erosion is obviously large, but at the locality from which the 

 materials for study were selected, the fresh rock was still overlaid by 

 several feet of residuary matter in the form of sand, gravel, and clay, 

 which constitute the prevailing soil of the region. Although very plastic 

 when wet, and inclined to cake during seasons of drought, this soil is 

 nevertheless fairly fertile and easily cultivated. 



Petrographic Features of the fresh Rock. 



In its least changed condition — the best material obtainable from avail- 

 able outcrops — the rock is a highly feldspathic gneiss carrying abundant 

 folia of black mica. Under the microscope it shows an extremely va- 

 riable, sometimes cataclastic structure. Both orthoclase and soda-lime 

 feldspars are present, the latter being often fairly clouded with enclosures 

 of minute zircons. These last are so small as to have their optical prop- 

 erties quite obscured by those of their host, but can be washed from the 

 residual sand in quantities and readily determined. The largest forms 

 obtained thus measured some .12 by .4 millimeters, the average being 

 perhaps .06 by .30 millimeters. Black mica is very abundant, as noted 

 above, and quartz occurs in the usual granular forms. There is the usual 

 sprinkling of iron ores and apatites, and in addition small garnets, and 

 in considerable quantity a soda-potash bearing zeolite occurring in dense 

 aggregates and radiating masses, almost wholly without action on light 

 and with crystal outlines so poorly developed as to make their true min- 

 eralogical nature, except by microchemical tests, a matter of surmise. A 

 bulk analysis of the rock and also one of the portion decomposed by 

 boiling hydrochloric acid is given in columns 1 and 2 below. 



Physical Manifestations of Weathering. 



In weathering the rock becomes brownish and friable, passing into the 

 condition of a loose agglomerate of angular gneissic fragments, so discol- 

 ored by decomposition as to be scarcely recognizable, and imbedded in 

 fine brown sand and dust. Finally, on the immediate surface the rock 

 has weathered to a red, tenacious clay, which still retains a sufficient 

 quantity of undecomposed silicates and of free quartz to have a distinct 

 gritty feeling when rubbed between the hands. Under the microscope 

 it shows sharply angular fragments of quartz and feldspar, numerous 

 folia of black mica, and much opaque or amorphous matter so stained 

 by iron oxides and decomposition products as to quite obscure its true 

 mineral nature. The state of mechanical subdivision of the material is 



