374 KEYES — GRANITES AND PORPHYRIES IN THE EASTERN OZARKS. 



of time this cycle was a later one than that represented by the Tertiary 

 peneplain and immediately preceded the present one. The effects of the 

 present cycle of erosion are well shoAvn in the sharply cut trenches in 

 the plain, where erosion has recently been accelerated with vigor. 



Relations of principal Types of acid Rocks to physiograi-hic 



Provinces. 



Recently topographic maps have been made of a portion of the region 

 on a scale of one inch to tlie mile and witli a contour interval of 20 feet. 

 These sheets have afforded means of accurate comparison of different 

 parts of the district which were previousl.y not available. In the four 

 occupied b}^ the Farmington, ]\Iine la Motte, Iron ^Mountain and Bonne 

 Terre districts, two marked physiographic provinces have been distin- 

 guished, as already stated. There is a highland plain and a lowland 

 plain. The great granite mass of the region lies entirely within the low- 

 land plain. To the eastward it is covered by the limestone, which forms 

 the drainage divide between the Saint Francois and Mississippi river 

 systems ; but beyond the ridge the granite again appears in some of the 

 stream beds, as at Jonca (see plate 17). 



From the relationship established between the large granite area and 

 the lowland plain of denudation, the inference is that at this point in the 

 Ozark uplift the surface facies of the granitic mass has been removed 

 through erosion, in part perhaps in pre-Cambrian times, but largely 

 at a very recent period. The fact that the surface facies has been actu- 

 ally eroded is shown by several high, isolated hills which rise out of the 

 granite area. Some of these, as Knol) Lick, for instance, are still capped 

 by porphyr3\ They clearly indicate that from the surrounding area 

 granite to a depth of over 400 feet has been removed in addition to the 

 surface shell of porph^^ry. 



It is a noteworthy fact that the large granite area is at the extreme 

 eastern end of the Ozark crest. Not only are the three slopes fully ex- 

 posed to erosive agencies, but proximity to thcMississippi river brings 

 the extremes of altitude so closely together that the general erosion of the 

 region is at a maximum at this point. Attention has been latel}^ called 

 to the apparentl}^ modern date of the Ozark uprising and the very recent 

 acceleration of its growth, as clearly shown in certain physiograi)hic fea- 

 tures. The place of most pronounced elevation seems also to be in the 

 Saint Francois district. This and the unusually favorable conditions for 

 rapid erosion have been the direct cause of the removal of an amount of 

 material greater than in any other part of the uplift and the exposure of 

 rocks more ancient than at any other point. Three thousand feet would 



