38 



GKOLOGY. 



below the mouth of Arroyo Sombreretillo they make their appearance quite frequently. Again 

 below this point, they are still more plentiful. On the oyster-terraces, some forty miles below 

 Laredo and near the Eancho San Ignacio, there is a spot remarkable for the abundance of this 

 fossil; as are also the following jDlaces : the slope of "Ked Eidge," of "Shady Bluffs," and 

 Septariae Hills. Their most common shape resembles very much a small, flat loaf of bread. 

 Both on the out and inside large irregularly-shaped divisions, like cob-webs, are to be seen, 

 which seem to have been formed by a net-work of veins composed perhaps of crystals of gypsum, 

 which commonly abounds here. This fossil is in all probability to be referred to Indus hel- 

 montii, (turtle stones.) 



The largest septariae of this kind was seen at Laredo, whither it was brought from the 

 vicinity of the Arroyo Sombreretillo. The whole piece, 2 feet across and nearly 1 foot thick, 

 consisted only of the cob-webbed part, showing cellular aggregations of a silicious matter ; its 

 color was pale yellow. 



^i-- 





' The Bouibsbell Bluff," on the lower Rio Bravo del Norte, Texas. 



Fragments of similar septariae, if these latter deserve that name, were also found in several 

 other localities below Laredo. Other aggregations, septariae-like in character, are seen as balls 

 or nodules of various sizes, composed of a pale green sand imbedded in a similar but more 

 brittle marly matrix. Clusters of such balls abound imder some green sand bluffs near Eancho 

 Clareiio ; from the size of a three-pound cannon shot up to that of the largest bombshell — 

 sometimes entire, sometimes cracked in two. So striking and peculiar are these rocks, as to be 

 justly entitled to the name of " Bombshell Bluff," with which they were christened. 



