60 0. Meyer — Species in the Southern Old-tertiary. 

 The ano-les measured and calculated were : 















Measured. 



Calculated. 



a 



a 



b 



100 



yN 



010 



90° 



90° 



s 



/v 



s 



120 



A 



120 



95° 8' 



94° 58' 



a 



^ 



d 



100 



^ 



101 



38° 20' 



38 c 22' 



d 



^ 



e 



101 



s. 



111 



19° 52' 



19° 46' 



e 



^ 



e 



111 



yv 



111 



95° 5' 



95° 6' 



a 



^ 



e 



100 



^ 



111 



42° 25' 



42° 27' 



b 



^ 



k 



010 



/v 



021 



40° 45' 



40° 49' 



The plane of the optical axes is parallel to the base, one of 

 the bisectrices being normal to the macropinacoid as shown by 

 a polarizing microscope. Owing to the minuteness of the crystal 

 examined the divergence of the optical axes was not determined. 



The opaque crystals from Obsidian Cliff show the same forms 

 with the additional basal plane, c, but are mostly developed as 

 in figure 2. With the exception of the macropinacoid the 

 faces were too dull to give good reflections, and the forms were 

 identified by approximate measurements only. 



Similar crystals of fayalite are found in the lithophyses in 

 rhyolite from several localities within the Yellowstone National 

 Park, which will be fully described in the geological report on 

 the survey of that region now in progress under the charge of 

 Mr. Arnold Hague. 



These brief notes are published at this time for the reason 

 that well-developed, natural crystals of fayalite have not been 

 found before, so far as we know, though they frequently occur 

 as furnace products and in iron slags. Their natural occurrence 

 in this instance bears a striking similarity to the artificial, the 

 obsidian being as perfect a slag as any artifically produced, and 

 the association of tridymiteand prismatic quartz with the fayal- 

 ite in the closed cavities of lithophyses leaving little doubt as to 

 their igneo-aqueous origin. 



Art. X. — The Genealogy and the Age of the Species in the 

 Southern Old-tertiary ; by Otto Meyer, Ph.D. 



Part II * 



The Age of the Vicksburg and Jackson Beds. 



The table, given in Part I, indicates that the Jacksonian 

 stands between the Claibornian and the Vicksburgian, but it 

 affords no evidence as to which is the most recent formation ; 

 this requires a special discussion. 



The history of the age of the Vicksburg bed is closely con- 

 nected with the literature of the orbitoidic limestone in the 

 vicinity of Claiborne, as the following review shows. 



*Errata in table of Part. I. — Nos. 3 and 13. Transpose names under Claiborne and 

 Jackson. No. 1 7. Substitute Lea for Mr. after Astarte sulcata. No. 80. For protracta 

 read protractus. Add No. 85 under Jackson and Claiborne Terebra costata Lea. 



