70 A GEOIJOGICAI. SUBVET OF LA^'DS 



THE ANACACHO LIMESTONE. 



This is the heaTy-bedded yellow rock, which forms the Ana- 

 cacho mountains. It consists of strong ledges of limestone, 

 whose combined thickness measures about 400 feet. The rock is 

 frag-mental. It consists of broken particles of shells and plates and 

 spines of various organisms, such as molluscs, sea urchins, and 

 corals, and with these are mingled entire smaU tests of foramini- 

 fers. It may be called an organic sandstone, with some grains 

 that consist of minute entire tests of marine organisms. The 

 rock has been consolidated by the solution and introduction of 

 carbonate of lime as a cementing material. It must be regarded 

 as the material of a submerged bar, formed in the sea not far 

 from a shore. The material is well sorted and washed, but not 

 much worn. In many places the ledges exhibit a cross bedded 

 structure. This is especially frequent in the west end of the Ana- 

 cacho mountains, as in the west branch of Elm creek at a point 

 about one and one fourth mUe southwest of the Anacacho bench 

 mark of the U. S. Topographic Survey, where the following sec- 

 tion is seen in a nearly vertical waU of the limestone. 



Thickness 

 in feet. 



4. Heavy ledges of cross-bedded fragmental limestone 3i 



3. Thin-bedded limestone 9 



2. Single ledge of solid limestone 8 



1. Thin-bedded fragmental limestone 2 



Equivalence. 



The sudden thinning out oi the Upson clay against the west 

 margin of the Anacacho limestone and its entire disappearance 

 under its formation farther east, shows that this Umestone be- 

 gan to be fonned in the region of the Anacacho mountains before 

 the making of the Upson clay had come to an end farther west- 

 ward. We must hence conclude that the two are at least in part 

 of the same age. Stratigraphic relations indicate that the mak- 

 ing of the Anacacho limestone continued for some time after the 

 Upson clay had aU been deposited and that the former is the 

 equivalent not only of the later, but also of a part of the Eagle 

 Pass formation, presently to be described. 



Thickness and Distribution. 



The resistance of the Anacacho limestone to erosion has 

 caused it to remain as a capping of a plateau or low mountain, 



