ZONAL WEATHERING OF A HORNBLENDE GABBRO 



ALBERT D. BROKAW and LEON P. SMITH 

 University of Chicago 



In connection with a study of the alteration of the so-called trap 

 or diabase dikes at La Grange, Georgia, 1 one of the writers collected 

 some interesting specimens showing a weathered zone in which the 

 decomposition is extreme, with an abrupt transition into fresh rock, 

 practically free from the effects of weathering. This paper is con- 

 cerned with the description of a typical specimen, and with the 

 results of chemical analyses of the fresh rock, the partly altered 

 material, and the extreme phase of alteration present. 



La Grange is in the extreme western edge of the state, not many 

 miles from the southern end of the belt of crystalline rocks which 

 extends from Maine to Alabama. The country rock is for the most 

 part gneiss, cut by granitic intrusions. Both gneiss and granite are 

 cut by pegmatites and basic dikes, the latter usually called diabase, 

 and supposed to be of Triassic age. At La Grange there is a series 

 of dikes, only a few feet apart, ranging up to forty feet in width, 

 with an average of about four to six feet. The dikes have a general 

 north-south trend, and are abundant over an area of a little more 

 than half a square mile. 



The older crystalline rocks are very deeply weathered. The 

 basic dikes have been considerably altered at the surface, and in 

 many instances may be traced by the strong iron stain they have 

 imparted to the soil. In some excavations, however, material 

 showing no appreciable effects of weathering may be obtained. In 

 some case; weathering has been strongly marked along joints, 

 changing the character of the material for an inch or less, beyond 

 which the rock is fresh. Fig. i shows a specimen in which the zone 

 of alteration is about an inch thick. Analyses were made of the 



1 L. P. Smith, Alteration of Diorite by Weathering, Dissertation, University of 

 Chicago, 1915. 



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