368 KEYES — GRANITES AND POJIPHYRTES IN THE EASTERN OZARKS. 



spars are arranoed the same as in diabase. The })henocrysts are mainly 

 quartz, though feldspars are not of infrequent occurrence. 



COXSANQUiyiTY OF THE ACID HOCKS. 



Nowhere does a sharp line of demarkation exist between the porphyry 

 and the graliite. When closely associated the two kinds of rocks merge 

 into each other gradual!}'. The transition zone varies in different places 

 from a dozen to a hundred or more yards in width. It appears to afford 

 good evidence that in general the two rock masses represent not neces- 

 sarily numberless separate eruptions, as once believed, but for all practical 

 purposes a single one or comparativel}' few, with special differentiation 

 in the different parts. 



As already stated, the chemical composition of tlie granite and the 

 porphyry is the same. Their mineralogical constitution is likewise es- 

 sentiall}^ identical. Tlie chief difference discerni])le between them is 

 merel}^ in the size, arrangement and relations of the components. Since 

 the structural characters of igneous rocks are de[)endent largely if not 

 entirely upon the accidental pln'sical conditions under wliicli tlie molten 

 magmas have cooled and solidified, it is of })rime imj)()rtance in the con- 

 sideration of the relations of different phases of the same body of rock to 

 take into account the fundamental princi})les involved in the production 

 of the several facies. It is a well known fact that under ordinary cir- 

 cumstances the texture of a rock mass of igneous origin is glassy or very, 

 fine grained at the surface, and as the distance from its ex[)osed face 

 increases the grain becomes coarser. Granite is commonly regarded as 

 a deep seated rock, one which has solidified very slowly and under great 

 pressure. Porphyry is formed where the pressure has not been so enor- 

 mous and where the cooling has been retarded less. All the stages may 

 exist in the same mass, but in the very old rocks devitrification has taken 

 place in the glassy portions, and other secondary changes have usually 

 greatly obscured or totally obliterated the original features. Moreover, 

 degradation has often acted vigorously and erosion has removed much 

 of the original surface of the rocks. 



In the district under consideration just such conditions as those de- 

 scribed appear to have jirevailed. It has been observed that, with a few 

 exceptions, wherever the granite and porphyry occur together the former 

 occupies the lower levels. \\'itbin the larger granite areas, wherever 

 high, stee}) sided hills exist, the coarse grained rock gradnates upward 

 into fine grained varieties and finally into the typical aphanitic porphyry 

 which forms a protecting cap to the elevation. These gradations upward 

 of the granite into porphyr}^ are especially well shown in the case of cer- 

 tain isolated hills, and probabl}^ it would be found equally true in most 

 cases if proper means of observation were afforded. The ]:)rincipal granite 



