IOWA ACADEMY OF SCIENCES. 
29 
other tough stone, the blocks may be exceedingly large, some- 
times several hundred yards intervening between the different 
joint planes. Orogenic stresses may sometimes assist strains 
produced by contraction in cooling, and they operate largely in 
a single direction, most frequently horizontally, thus giving a 
well defined bedded aspect to the rock masses. The stratifica- 
tion ascribed to some crystallines may, therefore, be regarded as 
secondarily acquired, and not as a primary condition obscured 
through metamorphic action. 
Jointing and the development of pseudo-stratification lines 
may be regarded as the first stages in the process of dissolu- 
tion. (Plate ii. ) In the regions farther north, within the limits 
of the glacial boundary, where granite or other hard massive 
rocks occur, the surface presents, as already stated, a remark- 
able fresh appearance, all the effects of decay being strikingly 
absent. In comparatively recent times, geologically speaking, 
the glaciers removed all the loose material over which they 
passed, and the boulders from the old ledges in the far north 
now lie strewn over the surface of the country to the southward 
so far, approximately, as the Missouri and Ohio rivers. Beyond 
the line thus marked out ice invasions have not effected the 
rocks, which have long been decaying without serious interrup- 
tion. In the crystalline area of Missouri all of the various 
stages of disintegation are well shown from the solid uneffected 
mass to the incoherent granitic sand. 
In addition to the horizontal divisional seams there are cer- 
tain vertical ones nearly as prominent. They pass from granite 
to porphyry and other kinds of rock without regard to mass. 
As stated in the case of the horizontal planes of natural cleav- 
age, the origin of the joints is due in part to contraction of the 
igneous masses during the original cooling, and in part to sub- 
jection to severe torsion. The latter force is in all probability 
in action at the present time; for as will be shown in another 
place, crustal movements have taken place in very recent times 
and probably continue to the present. That systematic joint- 
ing may actually arise from strains of this kind, has been satis- 
factorily proven experimentally by Daubree*, and appears to 
find full confirmation in other extensive trials, as well as in 
the field. The most prominent jointing planes present a 
remarkable uniformity of direction. The results of a large 
^Etudes syn. de geol. Exper., p. 300. Paris, 1879. 
