191Jf\ Geology of Chapel Hill 31 



4. " Gravel," " Natural Sand-Clay," fragments of orthoclase and 



quartz with sand and clay 10 to 20 ft. 



5. Fragmental rock, partly decomposed angular fragments 2 to 



4 inches in diameter 10 to 15 ft. 



6. Fragmental rock, angular, much coarser and less decayed 



than in No. 5 5 to 15 ft. 



7. Solid rock, " Bedrock," " Country rock." 



THE PROBLEM OF ZONATION 



That some zonation exists in the igneous rocks of this vicinity 

 has been shown by Professor Collier Cobb.^ This problem, 

 however, has not been fully solved and there are several factors 

 which make its complete solution extremely difficult. 



The mantle rock in this area is twenty to fifty feet thick which 

 prevents the occurrence of many exposures of the solid rock. 

 This makes it very difficult to determine accurately the relations 

 and boundaries of the probable zones beneath. If wide dikes 

 of various igneous rocks, deeply covered, cross each other nearly 

 at right angles, the result would closely simulate zonation in 

 which the zones are somewhat circular. Such a resemblance 

 would be especially strong in this vicinity where half or more 

 of the zone circle is concealed beneath sedimentary and metamor- 

 phic rocks. These conditions, however, do not disprove the pres- 

 ence of zonation ; they have not been fully confirmed by observa- 

 tion in the field but are suggested by the occurrence of acidic 

 and basic dikes throughout the stock. 



ECONOMIC GEOLOGY 



Zone ISTo. 1 of the generalized section given above is the true 

 surface soil of the upland and l!^o. 2 contains the brick clay of 

 the area. Some of the clay used in the old brickyards of Chapel 

 Hill was taken from zone 3 and therefore unsatisfactory results 

 were obtained. Zones 1 and 2 have been largely removed by the 

 long continued erosion and neither now occurs in very large 

 areas in the vicinity. 



This leaves zone 3 at the surface over a large part of the area, 

 especially on the valley slopes, and Xo. 4 and its decomposition 



'Zonation in the Chapel Hill Stock. An address (unpublished) before the 

 Ellsha Mitchell Scientific Society, December, 1912. 



