SILICEOUS OOLITES IN PENNSYLVANIA 261 
much lime. They are usually in the form of flattened ovals and 
have been developed since the formation of the shale. 
Throughout the Ordovician limestone but especially in the 
lower and dolomitic beds of the system there are great numbers 
of chert and flint concretions. In some cases these bodies possess 
the forms of sponges, and although they have been so much altered 
that they cannot be definitely identified as sponges, they are 
believed to be remnants of these animals and it is believed that 
sponges originally supplied some of the chert around which most 
of these concretions were built up. Some of these bodies of chert 
possess a distinctly concentric arrangement, while others are very 
irregular and often very large. In a few cases these masses of 
quartz and chert are as much as three feet in diameter and appear 
to have been cavity fillings in the limestone. The silica varies 
in composition from flint and chert to rock crystal and in some 
cases gradations from chert to the phanerocrystalline variety in 
massive form may be observed in the same body. 
Of considerable economic importance are the beds of limonite 
in the transition series. This mineral forms concretions in the 
decomposed limestone beds where the limestone has been replaced 
and fills cracks in a brecciated sandstone which is interbedded with 
the limestone. These concretions are sometimes a couple of feet 
in diameter, and are frequently hollow with the central cavity 
filled with water. When they are broken an explosive sound is 
produced, showing that they exist under a state of tensional stress. 
The formation of these bodies probably began by the deposition 
of iron oxide around the walls of a cavity filled with water and they 
grew by replacement of limestone. 
The source of the iron is believed to be found in the pyrite in 
the shales and limestones of the Ordovician, in the red sandstones 
of the Silurian, and probably in the limonitic and sideritic shales 
and sandstones of the Carboniferous, which have been removed by 
erosion, leaving their iron contents to be carried downward by 
meteoric waters. The rapid solution of the interbanded calcareous 
sandstone and limestone strata permitted a slumping of the solid 
sandstone, thus producing an extensive sandstone breccia to be 
cemented with iron oxide. 
