io STUDIES IN GEOLOGY, No. 3 



and lignite mentioned in connection with Grzybowski's 

 sectiqn. Beneath bed i of his section are heavy pebble beds, 

 conglomerates, loose, friable sandstones, intercalated with 

 highly colored clays similar to the uppermost bed of the 

 formation. These brightly colored clays form the base of 

 the Variegated formation. 



The Lower Zorritos formation, which follows the Var- 

 iegated in downward succession, consists above of heavy, 

 massive, in places conglomeratic sandstones, normally light 

 yellow in color, but frequently stained red or brown by iron. 

 The lower part of the formation is locally very calcareous 

 and fossiliferous ; most of the molluscs hereinafter described 

 from the Lower Zorritos came from these lower horizons. 

 The basal Lower Zorritos sandstones are thin-bedded, with 

 abundant intercalations of gray to yellow sandy shales, 

 some of which are carbonaceous and contain plant fragments. 



The Heath formation, the lowest exposed in the district, 

 consists primarily of gray shales, which weather brown, 

 with light gray to yellowish gray sandy shales and thin beds 

 of loose, unconsolidated sands interbedded with dark gray 

 shales. The uppermost beds are the soft dark gray shales, 

 with carbonate nodules ; they contain much selenite and 

 weather to the characteristic reddish brown color, excepting 

 the vicinity of Sechurita and in Quebrada del Grillo, where 

 they weather to a pale green. Below the upper shaly beds 

 the sandy phase appears ; it consists of a thin-bedded alter- 

 nation of very loose and unconsolidated sands with clays 

 lighter in color than those mentioned above. At and south 

 of Quebrada del Grillo the sandy horizon is represented by 

 a bed of heavy conglomerate. The entire Heath series is 

 measurable in thousands of feet; its thickness is unknown, 

 the base not having been seen, but several thousand feet of 

 it have been observed on the coast south of Zorritos. The 

 Heath is the mother-rock of the petroleum mined in the 

 Zorritos 'region, and most of the notable production is 

 obtained from horizons in its sandy phase. The Zorritos 



