252 The Aincriciui (jeoloyisf. April, 1895 
Where the veins f)f chalcedon}^ occur alone, they are so per- 
fectly analogous in fornj and position with the dikes as to 
make it evident that the joints and cracks tliey occupy are of 
the same origin as those of the dikes, and they may throw 
added light on the method of the formation of these cracks. 
The veins run not only vertical and in all tUrections, as de- 
scribed by Hatcher, but have also an inclined position, cross- 
ing the nearly horizontal layers of cla}?^ at angles varying 
from 1)0^' to 45*^: no smaller angles were observed. From 
their hardness the}" resist weathering a great deal longer than 
the soft clays and stantl up in jagged lines above the surface. 
When the clay around is removed and the support fails, pieces 
are broken off from the thin seams and fall on the neighbor- 
ing clays ; thus wlu)le hills are covered with small sheets of 
quartz and are i)rotected as by a shingle roof from the action 
of rain. Other local elevations are determined by the union 
of several veins in a common center, and from these centers 
run ridges, more or less pronounced, each determined by a 
vein and with a backbone of chalcedony. When a vein enters 
a iiill obliquely to its stratification, it determines from its su- 
perior resistance to erosion the outer and upper tdg,e of a cliff 
on the hillside. 
When the veins meet and cross they do not penetrate and 
destroy each other, but fuse, and perfect homogeneous crosses 
were obtained from localities of such intersection. Frequent 
inclusions of clay and calcite are found in the veins, and the 
clay is uniformly from a layer below whei'e it is found. 
In seeking an explanation for these dikes it is evident we 
must seek for the cause of the cracks which they occupy, and 
in doing so we must consider the evidence furnished by the 
veins of chalcedon}^ as well as the dikes proper. AVe find that 
the theory pro))osed by Diller to ex])lain the dikes in (Califor- 
nia is here only partial!}' applicable. The conclusion reached 
by him that the sand wiis forced up from below with water, 
receives conclusive conlirmatory evidence from the dikes of the 
Miocene clays as shown above. On the other hand, the origin 
of the cracks receives no light from his explanation. The 
absolute lack of any parallelism in the cracks precludes the 
idea of their i)roduction by earth movements; nor were they 
produced by su<*cessive movements in different directions, as 
