S. Poivers — Gastropod Trails in Sandstones. 103 



size. Some distance east of this outcrop Dr. Udden 

 found exposures of the same Cieneguita series with 

 pebbles of limestone and quartz ranging "in sizes up to 

 three inches in diameter. . . . faceted in a manner similar 

 to the faceting of small boulders and pebbles in glacial 

 deposits" (loc. cit. p. 14). "Where observed by the writer 

 in another locality the conglomerate showed well rounded 

 quartz pebbles % to V2 inch in diameter which indicated 

 long transportation by running water. Numerous fos- 

 sils are reported from this formation by Dr. Udden. 



In Sierra Alta creek the upper beds of the series are 

 well exposed. An additional outcrop was recently noted 

 by Dr. Udden where the new road to Cieneguita ranch 

 climbs out of Cibolo creek. East of the creek along this 

 road there is an excellent exposure of Alta shales with 

 interbedded sandstone strata one inch to four inches 

 thick. The upper surfaces of the sandstones are covered 

 with trails like those here illustrated and described. 



Trails were found by Dr. Udden in a small limestone 

 ledge on the northern side of Sierra Alta hill (a conspic- 

 uous volcanic butte unnamed on topographic sheet) 

 associated with a Productus (near cord), a Chonetes, 

 and a Plenrotomaria. They were found by the writer at 

 the Morita creek exposure not far east of an old prospect 

 tunnel. No shells are associated with the tracks, but fine 

 limestone conglomerates about 20 feet lower in the section 

 at the last locality contain numerous bryozoa and small 

 tabulate corals. 



Description of the Trails. 



There are two distinct kinds of trails: a simple type 

 about 14 inch wide consisting of crescentic indentations 6 

 to 10 in a linear inch, and 1/20 inch deep (fig. 2) ; a com- 

 pound type % inch to 1% inches wide consisting of a 

 central groove from % to 5/16 inch wide with elevated 

 looped concentric ridges 1/20 inch high and % to y 2 inch 

 wide on either side (figs. 1 and 3). The central groove 

 in the second type appears to be similar to the simple 

 tracks where it is clearly shown. These two types never 

 appear on the same surface and only the first type was 

 observed at Morita creek. The compound type is con- 

 fined to sandstones whose surface is covered by minute 



