LOWER RTO JiHAVO. 37 



BLUE CLAY. 



Besides this limestone, there are strata of a dark gray, sometimes blue, clay, which either 

 cover or intersect the layers of green sand. This clay is often hard and rock-like, forming 

 sometimes extensive reefs and banks, which seriously obstruct the navigation of the Kio Bravo. 

 In other places, especially where it is under water, it is soft, and can be moulded between the 

 fingers like plastic clay, to which it is closely allied, if not identical with it. The renowned 

 rapids of the "Isletas," in the vicinity of the Mexican Presidio San Juan El Bautista, are 

 formed by this clay. Above this place, some 10 miles, are similar clay deposits covered with 

 shoals of oyster-breccia 2 feet thick, pieces of which were added to our collection of fossil 

 specimens. 



A clay similar to this usually accompanies the lignite coal that has been referred to above. 



These are, then, the characteristics in which the upper portion of the green sand belt differs 

 from the lower, where alternating strata of sandier compounds prevail, instead of the argilla- 

 ceous, as in the former. 



THE GREEN SAND WITH FOSSHj OYSTEES AND SHELLS. 



As the frequent occurrence of lignite coal characterizes the upper belt of green sand, so the 

 banks and shoals of fossil oysters indicate the lower. 



These oysters, together with conglomerated shoals of shells, seem to be of an age still later 

 than the fossils of the cretaceous period above ; some, indeed, would seem to belong properly to 

 the tertiary strata. 



The lower portion of this green sand belt appears generally as if it were constituted a part of 

 the eocene system. 



The prints and remains of dicotyledonous and monocotyledonous leaves and plants in the 

 lignite coal, with some fossil shells of later age, led to the conclusion that all such fossiliferous 

 strata belong to the tertiary period, which, however, seem to have only a local distribution 

 throughout. 



In this belt tertiary deposits occur very generally, sometimes constituting extensive tracts of 

 land. 



The fossil oysters and shells before spoken of are of the largest size ; and extensive banks of 

 them are seen at several places along that part of the river embraced by the lower green sand- 

 stone belt. Of these, we may instance Roma, the Island Las Ajuntas, Shady Bluffs, near Mier, 

 and a point some 25 to 30 miles above the mouth of the San Juan. Other similar fossils aro 

 often found at these places covered with a perfectly chalky-whi'ie coat. 



Slate-like sandstone often intersects the green sand, and forms at a number of points massive 

 walls of considerable extent ; the adhesive quality of the particles of this rock, however, is often 

 not very persistent. Its inside is very soft and readily crumbles, as is shown sometimes by the 

 disintegration of the outer coat. 



SEPTARIAE. 



Another peculiarity of this lower belt is the frequent occurrence of a variety of septariae. 

 The highest place in the valley where they were seen (probably only as drift) was between Elm 

 creek and Las Moras. Further down the valley, however, they are more abundant, and just 



