Raymond. — Correlation of the Ordovician Strata. 



PLATE 5. 



Fig. 1. Thin section of glauconitic limestone from Putilowa, Russia. The 

 darker grains are glauconite; note the extent to which they are 

 altered to opaque limonite, especially in the large grain near the 

 edge to the left. 



Fig. 2. Another section from Putilowa in which glauconite is somewhat more 

 abundant and less altered. 



Fig. 3. A thin section of limestone from the zone of Asaphus expansus (top 

 of the Walchow) in a quarry opposite Iswos, Russia. 



Note that all three of these slides show the limestone to be made 

 up almost entirely of fragments of fossils, largely trilobites, brachio- 

 pods, and cystids. All magnified about 10 diameters. 



