Fig. i. — Microphotograph of Girvanella problematica Nicholson. About X42. 

 After Rothpletz. 



Fig. 2. — Microphotograph of peripheral section of a silicified oolite grain from 

 basal Ordovician at McGregor, Iowa. About X45. 



Fig. 3. — Cross-section of another grain from the same locality. About X45. 

 Note the well-developed algal structure in the outer portion and the band showing 

 radial structure within this. The interior is not preserved. 



Fig. 4. — Imperfectly preserved oolite grain. About X45. The interior and 

 peripheral portions of the grain were replaced with dolomite, with obliteration of struc- 

 ture, prior to silicification. 



Fig. 5. — Silicified grain showing well-developed radial structure but with algal 

 fibers nearly obliterated. About X45. 



Fig. 6. — Another grain showing fine concentric structure but with no distinct algal 

 fibers preserved. About X45. 



