The Upper Palaeozoic Fossils of Missouri. — Rozvlcy. 355 
Ample.riis vcrmicnlaris, n. sp. 
Fig. 51. Side view, natural size, .showing nearly the entire length. 
Fig. 52. Another view showing the contracted cup, natural size. 
Chonopliyllum greeiiei, n. sp. 
Fig. 53. Side view of an unworn and nearly entire specimen, natural 
size. 
Figs. 54, 55, 56. Side views of worn specimens, showing the invaginat- 
ed tabulje. 
Aulopora longi, n. sp. 
Fig. 57. Side view of the type specimen, natural size. 
Aulopora amplexa, n. sp. 
Fig. 58. A specimen surrounding a platycrinus stem joint, natural size. 
Spirorbisf dubius, n. sp. 
Figs. 59 and 60. Two views of this peculiar fossil, natural size. 
Cyrtolites hcnnetti, n. so. 
Figs. 61 and 62. Front and side views of the type specimen, natural 
size. 
Trigcria ? currici, n. sp. 
Figs. 40, 41, 42. Brachial, ])edicel, and front views of a large specimen. 
X2. 
Diclasiita ? pcdiciilits. n. sp. 
Figs. 43. 44, 45. Brachial, pedicel and front views, x2. 
Niiclcospira barrisi? White. 
Figs. 46, 47, 48. Pedicel, brachial and front views, natural size. 
ORE FORMATION ON THE HYPOTHESIS OF CON- 
CENTRATION THROUGH SURFACE 
DECOMPOSITION. 
By Charles R. Kkvks, Des Moines, Iowa. 
In seeking a suilal)le explanation for tlie localization of ore 
materials from an assumed i^enerally diffused condition in the 
country rock, a number of recent writers have leaned towartls 
the idea that throui^h the surface decomposition and degrada- 
tion of the land the heavy materials in large part remain behind 
and tend to concentrate into ore deposits, the extent of which 
in anv particular instance is to be regarded in a measure pro- 
portional to the amount of general erosion which that region 
has undergone. 
This hypothesis has received special attention in the consid- 
eration of the lead and zinc deposits of the Mississippi valley. 
