338 Pampean Formation. part n. 



S. Gregorio, which extend westward of the Rio S. Lucia, 

 the lower half is formed of coarse sand of quartz and 

 feldspar without mica, like that now cast up on the 

 beach near Maldonado ; and the upper half of Pampean 

 mud, varying in colour and containing honey-combed 

 veins of soft calcareous matter and small concretions of 

 tosca-rock arranged in lines, and likewise a few pebbles 

 of quartz. This deposit fills up hollows and furrows in 

 the underlying sand ; appearing as if water charged 

 with mud had invaded a sandy beach. These cliffs 

 extend far westward, and at a distance of sixty miles, 

 near Colonia del Sacramiento, I found the Pampean 

 deposit resting in some places on this sand, and in 

 others on the primary rocks : between the sand and the 

 reddish mud, there appeared to be interposed, but the 

 section was not a very good one, a thin bed of shells of 

 an existing Mytilus, still partially retaining their colour. 

 The Pampean formation in Banda Oriental might readily 

 be mistaken for an alluvial deposit : compared with 

 that of the Pampas, it is often more sandy, and con- 

 tains small fragments of quartz ; the concretions are 

 much smaller, and there are no extensive masses of 

 tosca-rock. 



In the extreme western parts of this province, be- 

 tween the Uruguay and a line drawn from Colonia to 

 the R. Perdido (a tributary of the R. Negro), the 

 formations are far more complicated. Besides primary 

 rocks, we meet with extensive tracts and many flat- 

 topped, horizontally stratified, cliff-bounded, isolated 

 hills of tertiary strata, varying extraordinarily in 

 mineralogical nature, some identical with the old 

 marine beds of St. Fe Bajada, and some with those of 

 the much more recent Pampean formation. There are, 

 also, extensive low tracts of country covered with a 

 deposit containing mammiferous remains, precisely like 



