KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 51. N:o 3. 13 
Plant-bearing deposits near Lago Nan Martin. 
Geology. 
The region north of Lago San Martin is undoubtedly one of the most inter- 
esting in Patagonia from a geological point of view. It was in this district that the 
fossil flora was found which forms the principal subject of this paper. In order to 
facilitate the understanding of the occurrrence of the plant-remains and the geological 
relations of the plant-bearing beds, it will be found necessary to give a short account 
of the geology of this region. 
Immediately east of Lago San Martin (text-fig. 2) there extends a large plateau 
or meseta measuring about 70 miles across from north to south and reaching, at its 
higher western border, in certain places a height of more than 1,800 m. above the sea. 
The meseta, which consists of sedimentary strata capped by a bed of basalt, slopes 
very gradually towards the east. North of Bahia de la Lancha, the eastern branch 
of the lake, this meseta comes into close contact with the eastern border of the cordil- 
lera. The boundary is roughly marked by the rivers Carbön and Fösiles, the former 
running in a northerly direction to Rio Mayer, the latter southwards to Bahia de la 
Lancha. These rivers have all but affected the separation of the meseta from the 
eastern slope of the Cordillera, only a short though high (1,360 m.) pass separating 
their sources. West of their valleys the eastern slope of the Cordillera is found to 
consist of the porphyritic tuffs which, from here on northward, play an important 
part in the structure of the eastern border-zone of the Andes (QUTENSEL 1911). On 
the east side of the valleys the meseta rises in steep barrancas presenting unparal- 
lelled sections of the thick sedimentary formations. Unfortunately fossils are rather 
rare in this series: HATCHER, who devoted some time to their exploration on the 
north side of the meseta, was unable to find any determinable fossils; and those 
collected by our expedition along its western border are too few and too scattered 
over separate localities to afford any basis for a satisfactory subdivision into differ- 
ent horizons. 
The completest section is found in the south, east of Bahia de la Lancha (text- 
fig. 3, which is a diagrammatical section of the western border of the meseta, at a 
on the sketch-map). No determinable plant-remains were found in this locality; but 
