142 Transactions of the Academy of Science of St. Louis 
Oklahoma to the Missouri River east of Marshall, Missouri. 
The Ordovician-Mississippian contact strikes northeast into 
Webster County, Missouri, where it turns and trends northwest 
nearly to the Mississippian-Pennsylvanian contact. The Missis- 
sippian rocks outcrop in a narrow band between the Ordovician 
and Pennsylvanian along the northern boundary of the Ozark 
Dome, indicating a steeper dip than that of the western side of 
the dome, a decrease in thickness of beds to the north, or a 
combination of these two factors. 
The rather abrupt boundaries of the uplift on the northeast 
and southeast suggest that the Ozarks owe their form not only 
to the intersection of the northwest and northeast structural sys- 
tems but also to the uplift of the whole region as a block. This 
uplift is also indicated by minor blocks within the main uplift. 
The boundary between the Ordovician and Mississippian in 
Greene County, Missouri, is marked by faults and folds cross- 
ing the Springfield anticline. These structures indicate an up- 
lift of the area of exposed Ordovician separately from the Mis- 
sissippian area, The gentle dips on the southwest border of the 
dome and the relatively steeper dips on the northwest suggest 
the action of pressure from the southeast, with the southwest 
side acting as a hinge. This is the same relationship as exists 
in the St. Francois Mountains block. 
St. Francois Mountains block. From Plate il it may be 
seen that the igneous rocks of the St. Francois Mountains form 
a rough square, the sides of which are oriented in northeast 
—southwest and northwest—southeast directions. The struc- 
tural nature of this block is not evident from the dip in the sur- 
rounding sedimentary rocks inasmuch as initial dips seen 
throughout the igneous area conceal any due to uplifting of the 
block. Evidence of the structural nature of the block is seen 
in the presence of faults parallel to the boundaries of the igneous 
rocks, in the trend of streams near the boundaries, and in the 
fact that the strike of the sides corresponds with the major 
structural trends of the state as a whole. 
