Tertiary Formations. 53 



of mammifera, which he terms the Pampean Loam (Limon 

 Pamp^en). The last is only covered by the deposits of the 

 present period. 



The Guaranian Tertiary formation is generally composed of 

 three conformable beds. The first consists of ferruginous 

 sandstone, often abounding in nodules of red oxide or hydrate 

 of iron, and ii. very beautiful agates (sardoines) of various 

 colours, whose angles are much rounded. It has a thickness 

 of upwards of 300 feet, where it is most fully developed. The 

 second bed, termed by M. d'Orbigny Calcaire dfer hydratS, 

 is a greyish-white argillaceous limestone, filled with harder 

 nodules, which are often very compact, with pebbles of 

 quartz, and with rounded grains of hydrate of iron. Its 

 greatest thickness is about 13 feet. The third bed constitutes 

 the upper part of the Guaranian formation, and is composed 

 of grey gypseous clay, filled with hard nodules. It is of the 

 same nature as the preceding bed, but contains no hydrate of 

 iron, that substance being replaced by a large number of no- 

 dules of gypsum disseminated in beds through the clay. Its 

 greatest thickness is about 13 feet. M. d'Orbigny found no fos- 

 sils in any of these three beds. All of them occur with great 

 uniformity in the province of Corientes ; and are not absolutely 

 horizontal, but, on the contrary, exhibit undulations and other 

 variations. The upper gypseous clays retain the water, and 

 thus there are produced, at the surface, immense marshes and 

 numerous small lakes, which give rise to one of the most re- 

 markable features of the topography of the country. Beyond 

 the great basin of the Pampas, M. d'Orbigny found the Gua- 

 ranian Tertiary Formation in the provinces of Chiquitos and 

 Moxos, and even between the 12th and 13th degrees of south- 

 ern latitude, near San Ramon and San Joaquin, and at the 

 fort of Beira. The points where it is apparent in the pro- 

 vince of Moxos seem to form part of a horizontal deposit, an 

 arrangement which leads to the belief that the Guaranian 

 formation levelled the inequalities of the surface before the 

 deposition of the Pampean formation which reposes on it. 



The second system of tertiary beds, termed by M. d'Orbigny 

 the Patayonian Tertiary Formation^ occupies a much greater 

 extent than the Guaranian tertiary formation. M. d'Orbigny 



