364 KEYES — GRANITES AND POKPHYRIES IN THE EASTERN OZARKS. 



of more than 3,000 square miles, in the southeastern part of the state. 

 They are all igneous in origin and pre-Cambrian in age. 



Important as the region is as a mining district, and as much as has 

 been done toward an inquiry into the nature of the mineral deposits, it 

 is somewhat remarkable that so little attention has been paid to decipher- 

 ing the stratigraphy of the region. Only very recently have the stratified 

 rocks begun to receive serious attention, while until lately, when their 

 consideration was taken up by Haworth * according to modern petro- 

 graphical methods, hardl}^ anything was known regarding the crystalline 

 rocks bevond their mere existence. 



General geological Features of the Region. 



Tlie crystalline area of southeastern Missouri is a highland district, 

 and constitutes what has been recently f termed the Saint Francois 

 mountain group. It forms in this state the eastern portion of the Ozark 

 uplift and is a part of the granitic foundation which has been ex}:)Osed 

 through profound erosion. Around the nucleus of ancient crystallines 

 the strata of the subsequently deposited formations are grouped in con- 

 centric belts of greater or less breadth. The cr3^stalline nucleus is com- 

 posed almost entirely of granites, porphyries, and diabases. 



In many places an imperfect bedding is often noticeable in the por- 

 ])hyries and granites, which fact formerly gave rise to the belief that 

 these rocks were sediments which had undergone metanior])hism to such 

 an extent that they were now in the last stages of the 2)rocess. It has 

 been recently demonstrated that the apparent lines of sedimentation are 

 in reality pseudo-stratification planes, and that they have been developed 

 in a way that is widespread among rocks which have cooled from molten 

 magmas. 



The continuity of the massive rocks is interrupted by numerous lines 

 of fracture. Most of these are merely joint-planes, many are slight ftiult 

 lines, while still others have the walls spread apart, the space being filled 

 with basic material forming dikes, which are sometimes of considerable 

 breadth. 



The joint-planes form several different series. Great uniformity in 

 direction is exhibited by those belonging to the most prominent set. 

 Slight deviations occur, but the general trend is north 60° east. The 

 second series makes angles of 80 and 100 degrees with the more i)romi- 

 nent one. 



* Missouri Geol. Survey, vol. viii, 1895, pp. 80-222. 

 t Missouri Geol. Survey, Iron Mountain Sheet, 1894, p. 4. 



