PARTING QUARTZITE SERIES. 19 
while the dolomitic mud in which these fragments are embedded is prob- 
ably detrital, and hence must have been derived from the subaerial erosion 
of some portion of the beds already formed. These things show that at 
the end of the deposition of the calcareous sediments of the Silurian a 
widespread but uniform elevation took place, so that those portions of the 
sea bottom which were nearest shore emerged from the waters and became 
dry land; after which the arenaceous sediments were deposited in the 
waters of a shallow sea. Inquiry into the origin of the smooth, delicately 
tinted lithographic dolomite which forms the distinguishing feature at this 
horizon gives further light as to the character of the sea. This rock is 
associated with the quartzite in the most intimate manner, alternating with 
it in successive small bands, thus showing that the two were deposited 
under very nearly the same conditions. Further, the cryptocrystalline 
dolomite often occurs in the quartzite in the form of nodules or masses of 
irregular shape, which are inclosed, like the quartz grains, in a cement of 
detrital dolomite of entirely different character. These nodules of litho- 
graphic dolomite inclose grains of detrital quartz similar to those in the 
rest of the rock, showing that either the d lomite was formed later than 
the deposition of the quartz or it was in a soft and plastic condition at— 
the time of sedimentation. The occurrence, often only a few inches away, 
of narrow and continuous bands of the lithographic dolomite containing 
very little quartz, which alternate with the typical dolomitic sandstone or 
quartzite, proves that the two varieties of rock were contemporaneous in 
formation, and so the second alternative must be accepted. The occasional 
occurrence of this rock in irregular blotches and lenticular interbedded 
nodules, which resemble in form and habit flint nodules (see fig. 3), is 
another significant feature; and these things point to the origin of the 
rock as a direct chemical precipitate from the waters of the sea. Such a 
precipitation of calcite is now going on in many places, such as the Ever- 
glades of Florida, where shallow land-locked waters are exposed to evapo- 
ration. During such evaporation the carbonate of lime brought into the 
lagoons by the streams which enter them is gradually thrown down and 
accumulates on the bottom. It is probable that the lithographic dolomite 
was thrown down as such a calcareous precipitate, and that its dolomiza- 
tion was accomplished later on, under the same conditions as have been 
described for the underlying massive blue dolomite, and very likely at 
