GEOLOGY OF THE SUN RIVER DISTRICT, MONTANA 557 



Reef. The fifth is an overthrust of the easternmost reef, the Car- 

 boniferous being found to overlie the Cretaceous. The reef west 

 of the Dam Site — Black Reef — ^consists of monzonite. So far as 

 known nothing has previously been published' bearing on the geology 

 of the Saypo quadrangle. 



The fossils were collected from five localities which are numbered 

 from west to east as on the accompanying map. 



Locality i. — ^At the big bend in the South Fork of the Sun 

 River just north of the junction of Goat Creek with this stream. 

 Embedded in a dark arenaceous shale were found: 



Inoceramus lahiatus Schlotheim, c.^ 

 Lingula sp. c. 



The horizon is probably Coloradoan Cretaceous. 



Locality 2. — -Immediately northwest of the junction of the 

 Sun River with its South Fork. The rock here is a shaly, fine- 

 grained sandstone. 



Pleuromya suhcompressa Meek C. 



P. suhcompressa wehberensis M. and H., r. 



Gryphaea calceola nebrascensis M. and H., r. 



Pieria sp. R. 



Ammonite, R. 



The horizon is Ellis (Jurassic). 



Locality j. — At the Dam Site, just east of the junction of the 

 Sun River with its South Fork. The rock is a dense brownish- 

 gray limestone. The fossils are quite thoroughly silicified. 



Syringopora surcularia Girty, c. 



Lithostroiion whitneyi Meek, c. 



Zaphrentis sp., c. 



Productus semireticulatus (Martin), R. 



Spirifer centronatus A. Winchell, C. 



The horizon is Madison (Mississippian) . 



Locality 4. — Arsenic Reef lies west of Big George Gulch. At 

 the Sun River valley it divides into two parallel ridges. On the 

 western ridge Professor Crosby noted Syringopora and Zaphrentis 

 in a hard limestone. It is therefore probably Madison in age. 



^C. indicates that the fossils collected are very abundant; c, abundant; r, rare; 

 and R, very rare. 



