194 Bulletin American Museum of Natural History. [Vol. XIV, 



thickness but there is no unconformity such as Logan seems to 

 have found in the Freezeout Hills. 



No. 22 is a bed of maroon colored clay with small limestone 

 concretions scattered through it, and is a most excellent horizon 

 indicator, for it is everywhere present, and its brilliant color 

 makes it a conspicuous feature for miles. At Bone Cabin it is 

 but 10 feet thick but increases to 30 feet in the Como Bluff. 

 The considerable amount of concretionary limestone would seem 

 to indicate brackish water. 



The maroon clay graduates into a bed of green clay with 

 similar small limestone concretions (No. 23). This layer like the 

 maroon bed is widespread and thicker as one goes south. 



The green clay with concretions is invariably covered with 

 smooth green clay (No. 24). In the Como Bluff this layer has 

 sandwiched into it a 4-foot bed of sandstone [p-^l)). The sand- 

 stone is of interest as marking the horizon at which the few known 

 Jurassic mammals were found. The mammal layer is the 6 

 inches of clay immediately underlying this sandstone. Most of 

 the American Jurassic mammal remains thus far found have 

 come from one quarry,' worked most successfully by Marsh and 

 later by the American Museum. This pocket seems to be 

 exhausted. 



Above 24 is a constant layer of large nodular limestone con- 

 cretions (No. 25). Single concretions may occur anywhere in 

 the clay but this layer and No. 27 are definite layers of almost 

 uninterrupted limestone nodules. The concretion layers are of 

 considerable importance as, when weathered out, they make a 

 prominent line on the bluffs just above which the most important 

 bone bed occurs. In this limestone layer Logan found five 

 species of fresh or brackish water mollusks." 



No 27 is a green clay of varying thickness in which the most 

 numerous quarries are located. On weathering, or having been 

 dried out, this clay breaks into angular fragments and is there- 

 fore known as 'joint clay.' The 'Nine Mile Quarry,' the 

 ' Aurora 1900 Quarry,' the ' Diplodocus 1897 Quarry,' and sev- 

 eral of Marsh's quarries are in this layer. Most of the fossils 

 are Diplodocus and Brontosaurus S]:)ecies. 



' See PI. XXVI, the S.E. corner of the map. 



* Unto knighti L., U. ivillistoni \,., U. baileyi L., Valvata teei L., and Planorbis veter- 

 nus L. 



