THE CAMBRO-SILURIAN QUESTION. 525 
certain periods quite marked. Some of these changes in eleva- 
tion are clearly defined, but the records of most of them are now 
obliterated. One of the most notable results of the warping of 
the lithosphere in this district was at the beginning of Paleozoic 
time, when the crystalline complex was subjected to profound 
subaérial erosion. Minor changes of level are also recorded. 
Another period of notable uprising was during Devonian times. 
A third was towards the close of the Lower Carboniferous, after 
the deposition of the St. Louis strata. Still another was one 
which closed Paleozoic deposition in the continental interior. 
Unconformities of greater or less prominence record these epi- 
sodes in the geological history of the region. In post-Pale- 
ozoic times the oscillations of level were manifestly not less 
marked than in the earlier periods. The most noteworthy per- 
haps was the gain of the land after the protracted submergence 
recorded by the Cretaceous. It was probably at this time that 
the forces of compression were felt most and that the warping 
and folding was more intense than at any other period in the 
history of crustal movement in the Ozark region. Moreover, it 
is to this period that the intrusions of igneous rocks along the 
southern or coastal margin of the uplift in central Arkansas are 
assigned. Evidences of a subsequent lower level of the land 
surface are manifested in the peculiarities of the topographic 
forms in the plateau district and in the relatively uniform even- 
ness of the upland plain. Conclusions deduced from glacial 
investigations point to a depression in very late geological times 
ot the surface ef the continental interior below its present posi- 
tion. This carries with it the inference that since the close of 
the Tertiary elevation has taken place. This is clearly indicated 
in the youthful topography now existing along the borders of the 
uplift. The watercourses have cut profound valleys in the gen- 
eral upland plain, and now flow in canyon-like trenches which 
are ever deepening as the streams recede from their sources. 
Erosion is now going on vigorously. The rivers are carrying away 
the débris from their steep-sided banks as fast as formed, and 
are rapidly cutting lower and lower their confined and contracted 
