38 MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 



limestones about 50 feet thick including an oolithic bed 6 to 10 

 feet thick. 



(4) The remainder of the section, estimated at about 600 feet, 

 consists of yellow to grayish cherty and sandy limestones in thin 

 beds represented chiefly by fairly smooth slopes strewn with 

 yellow and reddish sandy and cherty fragments. 



Fossil collections were made at a number of places in the Fort 

 Hall formation. The following faunal discussion is contributed 

 by G. H. Girty: 



The Fort Hall formation might appropriately be called the Aviculi- 

 pecten idahoensis zone, for it is particularly characterized by that species, 

 which occurs in most of the collections and in many of them is very 

 abundant. With A. idahoensis are associated a few other types of pelecy- 

 pods, among which a large Bakewellia or Pteria and two or three species 

 of pectinoid shells are the most common. There is also a form resem- 

 bling Myalina (possibly the Volsella platynota of the Ross limestone, but- 

 smaller and less abundant), and several types which are too poorly 

 preserved to be identified but in general expression suggest Myacites, 

 Schizodus, and Pleurophorus. A small naticoid (Nalica lelicfi) is 

 rather abundant in places, but otherwise, gastropods are practically 

 absent. 



In contrast to the preceding formation, the Fort Hall does not con- 

 tain any cephalopods nor, with the single exception noted below, any 

 brachiopods. As regards the pelecypods, the pectinoid shells, except 

 A. idahoensis, are much less abundant in the Fort Hall formation, and 

 some of the species of the Ross limestone appear not to occur there at 

 all. On the other hand, A. idahoensis appears to be restricted to the 

 Fort Hall formation. 



One collection shows a remarkable and interesting variant of the 

 Fort Hall fauna. It is distinguished by the absence of most of the 

 pectinoids, even of A . idahoensis, and by the abundance of terebratulas, 

 of which there are four or five varieties or species. Of the pelecypods 

 the most noteworthy are a large Lima (n. sp.) and a sharply plicated 

 oyster, besides which there are two species of Myacites?, a large Bake- 

 wellia? and one or two other forms. The gastropods are represented 

 by Natica leila and by another species, possibly a Pleurotomaria. 



Portneuf limestone. The Portneuf limestone is named from the 

 Portneuf River, at the head of which the limestone is well ex- 

 posed. The rock is a massively bedded, siliceous, and cherty, 

 gray to yellowish limestone. The chert occurs in rounded and 

 elongated nodules and in streaks. Silicified fossils, including 

 Spiriferina n. sp. (?), Terebratula semisimplex, and other tere- 



