SILICEOUS OOLITES IN PENNSYLVANIA 267 
silica 3.70 to 95.83 per cent and calcium carbonate from 88.71 to 
3.45 per cent—thus a gradation from calcareous to siliceous 
oodlite. 
I . II III A Single Odlite 
Per cent Per cent Per cent Per cent 
SIO Nae ee se isos: 3.70 56.50 95.83 SiOz 99.99 
Hez@sqSAN Oya cade sie: eA I.50 2.93 FeO 00.01 
Cal COT er as 88.71 16.84 CaO 1.93 
IMIG CO) cigs ctace Benge aco 8.09 2.60 MgO trace 
Waterers reeves rh at LE ye i ten 
This alteration is well illustrated in Fig. 5 which shows vari- 
ous stages in the process of replacement. In most cases the re- 
placement begins around the sand grain in the center, if one 
be present, due probably to the chemical influence of the silica 
already present in the odlite; but if there be no sand grain as 
a nucleus this process is more likely to begin in one of the outer 
rings of the concretion. A number of slides having been examined 
in conjunction with field observations, there remains no doubt 
that these siliceous o6lites originated by replacement of the 
calcareous forms. 
THE SOURCE OF THE SILICA 
The source of the silica is to be found in the cherty remains of 
sponges and other animals and in the sandstones. According 
to Van Hise, the organic silica is more readily dissolved than 
the mineral silica and it is true that in this region the crypto- 
crystalline variety of the mineral has been much more largely 
transported than the phanerocrystalline. There are good ex- 
amples of the solution of chert in a brecciated chert bed in 
which the fragments have been rounded off and silica deposited 
along the fractures as cement. In the sandstones there are 
numerous examples of the partial solution of sand grains and the 
movement of the silica along cracks in the rocks. This feature 
is well illustrated by Figs. 6 and 7, where sand grains may be 
