376 KEYES GRANITES AND PORPHYRIES IN THE EASTERN OZARKS. 



case, the normal and unchanged granitic rocks of the Saint Francois region 

 probably do not belong to the Archean at all, but to that recently estab- 

 lished S3^stem which represents the enormous interval of time between the 

 formation of the truly Azoic rocks and deposition of the lowest Cambrian. 



Summary. 



Recapitulating briefly, it may be stated that — 



1. The granites and porphyries are very closely related genetically, and 

 are to be regarded as fades of the same magma. 



2. Whatever may have been their origin, whether from a few or many 

 points of extravasation, the present relations of the two are that the 

 porphyry is an U[)per and surface facies of tlie granite. The thickness of 

 the former is variable, having been original!}' unequally developed in 

 different places and subsequently modified by both ancient and recent 

 erosion. 



3. The present geographic distribution of the granites and i^orphyries 

 is the outcome of very recent changes in the topographic configuration 

 of tlie region, and is not of ver}^ ancient origin, as has been usually con- 

 sidered to be true. 



4. The existing areal relations of the principal masses of the acid rocks 

 may be traced directl}^ to the S3^stematic and wides})read effects of recent 

 orogenic action upon the physiogra])hy. 



5. An element of uncertainty regarding the geological age of the mas- 

 sive crystalHue rocks now prevails, and the determination of their exact 

 age may perhaps always remain an unsolved problem. 



6. The basal complex of Archean schists exists in jMissouri within a 

 very moderate distance beneath the highest Paleozoics. It differs widely 

 in lithological characters from the crystallines of the state, whicli have 

 been usually referred to that age, but clearly approaches the more typical 

 Archean rocks of other districts. 



