Sandstone Dikes. — McCallie. 201 
near where the larger dikes occur, is to be seen during the low 
stage of water, an outcropping of sandstone which appears to 
be the exposures of larger dikes than any appearing along the 
river banks. 
The material of the various dikes is quite uniform both in 
physical structure and in mineralogical composition. It may be 
described as a light gray, fine-grained sandstone, containing 
considerable muscovite and some clay, the latter mineral form- 
ing a cement which binds the sand particles into a compact 
mass. In some instances, the sandstone has a yellowish or 
brownish color, due evidently to the presence of oxide of iron. 
These colors sometimes alternate with the gray color, thus pro- 
ducing a banded appearance with the bands running parallel 
with the sides of the dikes. 
The individual sand granules are sharp and angular show- 
ing but little or no evidence of having been long subjected to 
the wearing action of the waves or running water. They con- 
sist almost entirely of quartz containing little or no coloring 
matter. The mica which is irregularly distributed throughout 
the sandstone occurs in small, thin flakes having a silver-like 
lustre. In no instance does the mica exhibit any evidence of 
being arranged in layers, but on the contrary it occurs pretty 
evenly scattered throughout the sandstone dikes quite similar 
to the occurrence of mica in alluvial sands. The scales of mus- 
covite have flat surfaces and show no indication whatever of 
having been crushed or subjected to pressure since their orig- 
inal deposition in the dikes. 
One of the most interesting features of these dikes, a pecul- 
iarity distinguishing them from the sandstone dikes of California 
described by Dr. J. S. Diller,* is the presence of fossils. These 
fossils which consist mainly of bi-valves are frequently met 
with in more or less abundance in all of the larger dikes and in 
many instances form beautiful casts showing very perfectly 
both the external and the internal structure of the shell. The 
arrangement of the shells in the dikes is very irregular and like 
the scales of mica, shows no evidence of stratification or bed- 
ding. The shells occurring in the dikes appear to be identical 
with the fossils found in the enclosing Cretaceous clavs. some 
of which are made up largely of the casts of bi-valves. 
* Bulletin Geological Society of America, vol. i, pages 411-442. 
