246 Barbour and Torrey — Microscopic Structure of Oolite. 



Figure 21. — Cardinal view of ventral valve, from right side of specimen figure 



22. x2. 

 Figure 22. — Interiors of two ventral valves. Left specimen shows septum and 



muscular scars. Center of right hand valve has been removed, 



exposing shell of Plaiyceras. x 2. 

 Figure 23. — Cardinal view of specimen, showing pedicle-sheath, dorsal callosity, 



hinge area and numerous spines, x 2. 

 Figure 24. — Shell of Platyceras equilaterale, H., on tegmen of Ollacrinus tuberosus 



(not represented) showing numerous attached specimens of this 



species in various conditions and stages of growth. Natural size. 



Keokuk group, Crawfordsville, Indiana. 



Art. XXXIY. — Notes on the Microscopic Structure of Oolite, 

 with analyses; by Erwtn H. Barbour and Joseph 

 Torrey, Jr. 



While making notes the past autumn and winter prepara- 

 tory to a thorough study of the oolite of Iowa, several points 

 were noted which seemed to the writers to be of sufficient 

 interest to be presented here. Ordinary oolite occurs in this 

 State in deposits of considerable extent, but it is not proposed 

 at this time to do more than to describe the microscopic struc- 

 tures and to give analyses of two forms. The following are 

 analyses of two varieties of the Iowa river oolite : 



1. 2. 



Oolite with concretionary structure. 



Oolite with brecciated granules. 



Silica 



Oxide of aluminium .. 



Oxide of iron 



Calcium carbonate 



Magnesium carbonate. 



Specific gravity 

 l. 



2-iO 



3-20 



85-99 



8-52 



99-81 

 2-619 



Silica 



Oxide of aluminium .. 



Oxide of iron 



Calcium carbonate 



Magnesium carbonate. 



Specific gravity 



2. 



4-56 

 3-40 



84-33 



7-50 



99-79 

 2632 



Figs. 1, 2. — Micro- sections of Iowa River Oolite; magnified 10 diameters. cj 

 1. Showing concretionary structure. 2. Showing granular structure of spherules. 



