o70 KEYKS — GRANITES AND PORPHYRIES IN THE EASTEKN OZARKS. 



morphosed sediments, as was once supposed. Tlie most important ob- 

 servation is that in general the porphyries and granites were formed about 

 the same time, and that consequently neither can be regarded as younger 

 than the other. "^ Numerous cases are noted in which the complete transi- 

 tion from one facies to the other takes place within a very short distance. 

 The conclusion is also reached that there are — 



" Good reasons for believing tliat nuin\' of the prominent porphyry hills are the 

 results of individual outbursts," and that they "are formed of the lava which 

 reached the surface and formed variously shaped volcanic mountains or hills and 

 monticules, the lava of which would connect directly with the deeper seated ma- 

 terial." f 



Still more recently I the problem of the present distribution and rela- 

 tions of the granites and porphyries have been considered anew, witli 

 special stress placed upon the physiographic evidences, and very sug- 

 gestive results have been obtained. 



General a real Distribution. 



Stripped of its various thin patches of sedimentaries the main body 

 of massive crystallines forms a broadl}^ oval area about 50 miles long. 

 Of this the granite occupies about one-fourth, but is confined almost ex- 

 clusively to a single large mass in the northeastern part of the district. 

 Beyond tlie limits of the chief porphyry area there are numerous mounds 

 which rise out of the general limestone field. Outside of the main granite 

 body there are also other localities where the coarse grained rock occurs, 

 but the areas are small and rarely exceed a superficial extent of one 

 square mile. As incidentally observed by Haworth and others, the gran- 

 ites as a rule occupy the lower ground when occurring near the porphy- 

 ries, but the full significance of the fact seems to have escaped notice. 

 Man}^ of the peculiarities and irregularities in the present surface dis- 

 tribution are without doubt due to })re-Cambrian erosion, but recent 

 erosive agencies have also modified extensively the effects produced dur- 

 ing the more ancient cycles. In this connection, however, it is unneces- 

 sary to go beyond the main features relative to the geographic distribu- 

 tion of the principal masses. 



Physiography of the Region. 



3rAIX CHARACTERISTICS. 



The crystalline district of Missouri forms the eastern end of the crest 

 of the Ozark uplift. It is a semialpine region, with prominent solitary 



* Missouri Geol. Surve}% vol. viii, 1895, p. 218. 



tibid., p. 219. 



J Missouri Geol. Survey, vol. ix, 1895, pt. iv. 



