The Fauna of the Burlington Limestone. — Rowley. 245 
MaxinuDii angle of extinction in the zone of symmetry per- 
pendiciilar to g" (010). The mineral is so coarse that an in- 
definite number of thin sections would be necessary to deter- 
mine accurately the maximum equal extinction in the zone 
perpendicular to _^' (010), but it is certainly not essentially dif- 
ferent from that found in the labradorite of the diabases 
(namely 38°). 
To sumniarize the results of the study of the optic proper- 
ties of labradorite, we have found that the indices of refringence 
are higher than those determined by Fouque on plagioclase of 
the same chemical composition from the volcanic rocks. The 
maximum birefringence likewise is greater, but these two dif- 
ferences are wholly negligible when compared with the dif- 
ference in the values of the optic angle. The value found in 
the Minnesota type seems to indicate that the optic angle is 
distinctly greater in deep-seated than in volcanic rocks. But 
more measures are needed to establish this beyond question. 
The same is true of the greater extinction angle observed per- 
pendicular to ;/g ; the other extinction angles present no 
marked diflferences. 
{To be continued.) 
NOTES ON THE FAUNA OF THE BURLINGTON 
LIMESTONE AT LOUISIANA, MO. 
By R. R. Rowley, Louisiana, Mo. 
Blastoidea. 
Perhaps no other known Burlington locality is so prolific 
in and yields such a varied fauna of blastoids as Louisiana. 
There are twenty valid species and a number of well marked 
varieties. 
The commonest species and one of greatest range is Crypt- 
oblastus melo which is found sparingly from the very bottom 
of the I St., through the 2nd., 3rd. and 4th. to the very top of 
the 5th division of the Lower Burlington, beingiplentiful oiijy 
near the top of the 4th. division, where in a narrow band, it is 
often abundant. The best specimens come from the white 
limy cherts and the beauty and perfection of many of these are 
beyond description and can be appreciated only by being seen. 
