REVIEWS—GEOLOGICAL SURVEY OF IOWA. 197 
stances, so’attenuated as to be scarcely recognizable, more especially 
in a district deeply covered, like that of the greater part of Lowa, 
with Drift and modern deposits. It is to this attenuation of the 
strata, as well shewn by Professor Hall, that the comparatively 
subdued aspect of this western country isowing. The united strata 
are not sufficiently thick to admit of the production of any strongly- 
marked features, by either denudation or ordinary disturbing forces. 
Where anticlinals exist, they occupy low levels; and the only real 
elevations of the district have been produced by denuding agencies 
on undisturbed or nearly horizontal strata, where these, under special 
conditions (as in the case of the Niagara Limestone) have presented 
a more than ordinary thickness to the denuding force. On compar- 
ing these results with the phenomena exhibited in the district of the 
Catskill Mountains and the Appalachians, where the diminished strata 
of the West occur in accumulations of vast thickness, our author 
appears inclined to refer the general production of mountain chains, 
more to the action of denudation, than to that of elevating or dis- 
turbing forces. But, in this, his views are surely pushed too far. 
That denudation has produced mountain masses amongst undisturbed 
strata, as in the Catskill district, in the old red sandstone country of 
the Western Highlands of Scotland, and elsewhere, all the world 
must admit; and equally that anticlinals often occupy comparatively 
low leveis, as the beds of rivers, &c.: but when we extend our 
survey to the great mountain systems of the Earth—the Andes and 
their prolongations, which brim the eastern contour of the Pacific, 
the towering Himalayas, the Alps, and other chains, it becomes 
manifest that elevation has been there produced by disturbing 
agencies of no ordinary intensity. The bare occurrence of highly 
inclined and vertical (and sometimes even of reversed) strata, as seen 
in all these mountainous districts, the presence of stupendous 
voleanoes in many of them, with other well-known phenomena, point 
incontestibly to this fact. Denudation may have been concerned, 
and largely, in the excavation of valleys amongst these, in the pro- 
‘duction of lines of escarpment, and so forth; but denudation has 
there played the part of a mere secondary agent. The great views 
of Elie de Beaumont, however exaggerated and extended beyond their 
legitimate limits by some geologists, are still in their main features 
undoubtedly worthy of our reception; and these views are based 
essentially on the formation of mountain chains by elevating forces. 
