570 RALPH ARNOLD 
rate of plutonic rock material. After the ordinary process of erosion 
uncovers a sufficient surface of the indurated rock the dome struc- 
tures are formed by a successive scaling-off of blocks, through the 
development of cracks approximately parallel to the steeply sloping 
surfaces. ‘These cracks are probably due to expansive force devel- 
oped by chemical reactions (weathering) produced largely by moisture, 
the moisture passing upward by capillarity through the incipient 
cracks caused by expansion, and thus advancing the process. The 
cracks originate in positions advantageous to the accumulation or 
retention of the moisture producing the weathering, such, for instance 
as that occupied by the detrital material at the base of the slope or in 
the angle between overhanging blocks and the new dome surface. 
The direction of the cracks is determined by the configuration of the 
rock surface, being approximately parallel to it, the departures from 
strict parallelism being of such a nature as to omit angles and other 
features of irregularity. This parallelism to the surface is due to the 
expansive force acting along lines of least resistance, which in this 
case are practically normal to the outer rock surface. The slope of 
the surface is the governing function in the removal of the scales 
because the components of gravity and of the expansive force tending 
to dislodge the separated scales is greater on steep slopes than on low, 
while the component of gravity tending to counteract the expansion 
due to weathering is correspondingly less on steep slopes, becoming 
greater as the angle of declivity lessens. 
Fig. 8 illustrates graphically the two principal forces (expansive 
force due to weathering, and gravity) effective in dome formation. 
These forces are resolved into components at three different points on 
the dome surface, in order to make clear the last sentence in the. 
preceding paragraph. 
