lOO The American Geologist. Febraar.v, isfni' 
forms were discriminated from those of aqueous erosion. The 
recognized process has been that of scour, the action of which is 
downward and forward v/ith the glacial advance. Glacial scour and 
aqueous erosion are alike incompetent and as a rule inimical to the 
production of many forms. An unrecognized process is that of sap- 
ping, its action horizontal and backward. The tendency with glacial 
.icour is to produce sweeping curves and eventually a graded slope, 
while the tendency with the sapping process is to produce benches 
and cliflfs. Sapping is altogether dominant over scour. Under vary- 
ing conditions, however, its developing forms become obsolescent: 
their modification, theij, by rounding off of angles, puts them seem- 
ingly into the category of scour-forms. The ultimate effect is trunca- 
tion at the lower level of glacial generation. 
" The geology of the Yosemite National Park," by H. W. 
Turner, of the U. S. Geological Survey. The Sierra Nevada was 
first formed as a great mountain range by folding about the end of 
the Jurassic, the slaty and schistose structures being referred to this 
time. At a later period, probably during the early Cretaceous, sys- 
tems of joint planes were formed which intersected all of the older 
rocks of the range. When foimd in schistose rocks these joint planes 
cut the planes of schistosity at varying angles. During Cretaceous 
time the range was reduced in hight and the rocks which covered the 
granites were largely removed, especially in the southern portion of 
the range. When the drainage system of the Sierra began to be form- 
ed, the systems of joint planes exerted a strong influence on the 
courses of the streams. The much jointed zones disintegrated more 
readily, and because they were lines of least resistance, were more 
rapidly eroded and the tendency of the streams was thus to form can- 
yons along the shattered zones. By the end of the Tertiary the range 
had been reduced to one of comparatively gentle slopes. In the north- 
ern Sierra Nevada enormous volcanic eruptions filled the river chan- 
nels with lavas, so that the rivers were displaced and cut new chan- 
nels; but this was apparently not the case with that portion of the 
range from Yosemite valley southward. Here the Pleistocene streams 
appear merely to have deepened the Tertiary channels. The region em- 
braced in the Yosemite National park is exceptionally well exposed, 
hundreds of square miles being nearly devoid of soil or talus. The 
influence of the joint systems on the minor drainage can thus be seen 
to great advantage. With the larger canyons, however, this influ- 
ence is less marked to the observer, but if the direction of the strongly 
developed joint planes be plotted on a map, the canyons will usually 
be found to have similar courses. The Merced river is thought to 
have already excavated a shallow valley over the site of the present 
Yosemite valley at the end of Tertiary time. After the late Tertiary 
or early Pleistocene elevation of the range, the rivers cut the present 
deep canyons. From the illustrations shown it was clear that the 
granite about Yosemite valley is intersected by a set of very marked 
vertical joint planes, the course of which is parallel to the general 
