2 oo The American Geologist. ° ctober - 1903 - 
attain their greatest hight. This, however, is probably due to 
their better exposure at these points rather than to any other 
cause. 
The dikes vary in size from a fraction of an inch to two feet 
in width, and cut the enclosing beds of clays at various angles. 
At some points, they conform with the bedding of the clays 
and therefore resemble interstratified layers of sandstone, but 
more frequently they cut the beds of the clays at considerable 
angles and often stand nearly perpendicular. The beds of Cre- 
taceous clays themselves which are cut by the dikes are often 
somewhat disturbed and occasionally dip at an angle of from 
io° to 1 5°. Such disturbance as this is of rare occurrence 
along the Chattahoochee river where the strata are usually 
nearly horizontal or at most dip only a few feet per mile. 
Some of the individual dikes are very variable in width as 
well as in the direction of their strike. It is not uncommon for 
the larger dikes to increase to more than twice their usual width 
within a short distance and also to change their course, thus 
assuming an irregular or zig-zag direction. The smaller dikes 
on the other hand are generally more uniform both in their 
width and their strike. Sometimes the lesser ones continue 
for a hundred feet or more in a nearly straight line with little 
or no variation in width. It frequently happens that the dikes, 
which always have a jointed structure not unlike the rocks of 
the igneous dikes, divide into nearly equal divisions or they 
may intersect each other at various angles. More generally, 
however, both the smaller and the larger dikes are more or less 
parallel having a trend at an acute angle with the general 
course of the river. 
The sides of the dikes where they come in contact with the 
enclosing clays usually show very irregular or rough surfaces 
made up of numerous shallow depressions and their corre- 
sponding elevations. Where the elevations and depressions are 
wanting, the sides of the dikes occasionally exhibit smooth, 
well polished slicken-sides. These polished surfaces appear to 
be most abundant in the larger dikes. 
The depth to which the sandstone dikes extend is not 
known. They continue from the top of the river banks which 
in places are twenty or more feet in hight, to low-water mark 
below, with but little diminution in size. In the bed of the river 
