218 JOURNAL OF THE ARNOLD ARBORETUM (vol. ii 



The geological formations of tlie Ozark region are varied and represent 

 a wide range in time, but consist principally of the older Paleozoic sed- 



imentanes. 



Arcl 



of the Iron Mountain area in southeastern Missouri; surrounding these 

 and forming the surfa:^e strata over the greater part of the Ozark plateau 

 are the magnesian limestones, cherts and sandstones of the Cambrian- 

 Ordovician system. In the northwestern portions the purer limestones 

 and cherts of the Mississippian series appear, and similar beds of the same 

 age underlie a narrow strip on the eastern side, with sandstones alternat- 

 ing with the limestones southward. On the southwestern side in portions 

 of the Boston and Ouichlta Mountains tlie shales and sandstones of the 

 Pennsylvanian series predominate. Each of these formations has given 

 rise to a more or less clearly distinct topography, and has influenced in a 

 minor degree the character of the flora and the distribution of forest 

 species. The ancient crystalhne rocks rise in a series of isolated peaks 

 and domes from the surrounding country. Their areal extent is too 

 limited to support a very distinct ligneous flora, but on exposed faces and 

 slopes such arid and sterile conditions prevail as to exclude nearly all 

 plant life. Barrens, with a peculiar herbaceous flora, are sometimes found 

 on exposures where the slope is not too steep, and in the rubble and 

 detritus at the base, especially on the northern and eastern sides, consider- 

 able soil and moisture accumulates, supporting a varied ligneous as well 

 as herbaceous growth. Red Oak, Slippery Elm, June-berry and a small, 

 often shrubby form of the Red Maple are fond of such situations. Hick- 

 ories, Oaks and various other hard wood species occupy the more open 

 slopes and often attain a considerable size. The short-leaved Pine {Pinu^ 

 echinata Mill.) is rarely found on a few of the hills; and perhaps the most 

 marked feature of this section is the abundance of Vacciniums and allied 

 Ericaceae which thrive upon the acid soil resulting from the decom- 

 position of the igneous rocks. Rhododendro7i oblong i foil am (Small) Mill- 

 ais, reaches its most northern point in the Mississippi Valley here. The 

 areas of the magnesian and semi-crystalline limestones are characterized 

 by wide stretches of comparatively level uplands, with sheer escarpments 

 and perpendicular bluffs about its margins and bounding the deep V-or- 

 U-shapcd valleys, and by ridges and slopes deeply covered with frag- 

 ments of residual chert. This comprises the greater and most character- 

 istic parts of the Ozark plateau, and its floral features will be described 

 later. The magnesian limestones, cherts and quartzites of the Ozark 

 series underlie some of the most rugged portions of the Boston Mountains; 

 but where the sandstones and shales of the Pennsylvanian occur they 

 have sometimes been carved into low domes or canoe-shaped hills, stand- 

 ing out conspicuously from the broad surrounding valleys. 



The entire Ozark region lies in a zone of rather abundant rainfall; 

 the greater part of it receiving an annual average of between 35 and 40 

 inches, and the southeastern portion rather more than 40 inches according 



