34 2 P amp can Formation. part n. 



passes into a soft white sandstone with red streaks. 

 At the Calera de los Huerfanos. only a quarter of a 

 mile south of where I first met with the red sand- 

 stone, the crystalline white limestone is quarried: as 

 this bed is the uppermost, and as it often passes into 

 calcareous sandstone, interstratified with pure sand ; 

 and as the red sandstone likewise passes into soft white 

 sandstone, and is also the uppermost bed, I believe 

 that these two beds, though so different, are equiva- 

 lents. A few leagues southward of these two places, 

 on each side of the low primary range of S. Juan, 

 there are some flat -topped, cliff-bounded, separate 

 little hills, very similar to those fringing the primary 

 ranges in the great plain south of Buenos Ayres : they 

 are composed — 1st, of calcareous tuff with many par- 

 ticles of quartz, sometimes passing into a coarse con- 

 glomerate ; 2nd, of a stone undistinguishable on the 

 closest inspection from the compacter varieties of 

 tosca-rock ; and 3rd. of semi-crystalline limestone, 

 including nodules of agate : these three varieties pass 

 insensibly into each other, and as they form the upper- 

 most stratum in this district, T believe that they, also, 

 are the equivalents of the pure crystalline limestone, 

 and of the red and white sandstones and conglomerates. 

 Between these points and Mercedes on the Bio 

 Xegro. there are scarcely any good sections, the road 

 passing over limestone, tosca-rock, calcareous and 

 brio-ht red sandstones, and near the source of the S. Sal- 

 vador over a wide extent of jaspery rocks, with much 

 milky agate, like that in the limestone near S. Juan. 

 In the estancia of Berquelo, the separate, flat-topped, 

 cliff-bounded hills are rather higher than in other parts 

 of the country ; they range in a XE. and SW. direc- 

 tion ; their uppermost beds consist of the same bright 

 red sandstone, passing sometimes into a conglomerate, 



