320 Pampean Formation. paet h. 



Pampean mud. At the settlement of Bahia Blanca, 

 the uppermost plain is composed of very compact, 

 stratified tosca-rock, containing rounded grains of 

 quartz distinguishable by the naked eye : the lower 

 plain, on which the Fortress stands, is described by 

 M. Parchappe l as composed of solid tosca-rock ; but the 

 sections which I examined appeared more like a re- 

 deposited mass of this rock, with small pebbles and frag- 

 ments of quartz. I shall immediately return to the 

 important sections on the shores of Bahia Blanca. 

 Twenty miles southward of this place, there is a re- 

 markable ridge extending TT. by N. and E. by S., 

 formed of small, separate, flat-topped, steep-sided hills, 

 rising between 100 and 200 feet above the Pampean 

 plain at its southern base, which plain is a little lower 

 than that to the north. The uppermost stratum in 

 this ridge consists of pale, highly calcareous, compact 

 tosca-rock, resting (as seen in one place) on reddish 

 Pampean mud, and this again on a paler kind : at the 

 foot of the ridge, there is a well in reddish clay or 

 mud. I have seen no other instance of a chain of hills 

 belonging to the Pampean formation ; and as the 

 strata shows no signs of disturbance, and as the direc- 

 tion of the ridsre is the same with that common to all 

 the metamorphic lines in this whole area, I suspect 

 that the Pampean sediment has in this instance been 

 accumulated on and over a ridge of hard rocks, instead 

 of. as in the case of the above-mentioned Sierras, round 

 their submarine flanks. South of this little chain of 

 tosca-rock, a plain of Pampean mud declines towards 

 the banks of the Colorado : in the middle a well has been 

 dug in red Pampean mud, covered by two feet of white 

 sottish, highly calcareous tosca-rock, over which lies sand 

 with small pebbles three feet in thickness — the first 

 appearance of that vast shingle formation described in 

 1 M. d'Orbigny, ' Voyage, Part. Geolog.' pp. 47, 48. 



