18 T. G. HALLE, MESOZOIC DEPOSITS AND FLORAS OF PATAGONIA AND TIERRA DEL FUEGO. 
locality as belonging to the very base of division 6 and probably representing, in 
their lithological character, the transition between this division and the next lower 
one (5). — In this locality the strata are more disturbed than in the section at Bahia 
de la Lancha. The dip is on the whole towards the east, but the beds are often 
folded in low anticlines with a dip of some 20—30?. 
In the lower part of the section at c, marine fossils are very scarce and badly 
preserved. They consist mostly of bivalves, but there occur also very large casts of 
ammonites with a diameter of 30—40 cm. or more. (It is probably these huge fos- 
sils — which are poorly preserved and show no structural features — that have 
given rise to the name Rio Fösiles on the Argentine map.) Owing to the impossi- 
bility of carrying these specimens in the saddle-bags I could only bring home a frag- 
ment, which is, however, indeterminable. 
The majority of the fossil plants described in the present paper were found in 
this locality. They were collected at the very base of the section, partly in a light 
grey sandstone, but especially in a very hard siliceous slate of bluish grey colour. 
The latter rock was not found in a solid bed, the rock being quite broken up by 
frost-weathering into angular pieces remaining in situ on the gentle slope. Their 
abundant occurrence along a definite line parallel to the strike made it possible, 
however, to trace the bed for some distance, and a considerable number of specimens 
was secured. The slate evidently forms here only an intercalation in the sandstone, 
and the same species of plants occur in both rocks. The plants found in this locality 
were the following: Marchantites? sp., Nathorstia alata n. sp., Gleichenites cf. micro- 
merus (HR.), Cladophlebis australis (MORR.) SEW., Cladophlebis cf. Browniana (DUNKE.) 
SEW., Sphenopteris (Ruffordia?) Goepperti DUNE., Sphenopteris patagonica n. sp., 
Asplenites lanceolatus n. sp., Ptilophyllum acutifolium (MORR.) OLDH., Baiera cf. australis 
M”CoY, ÅArthrotaxites Ungeri n. sp., Elatocladus sp. 
Of the few invertebrates collected in more or less close connection with the 
plants, none has as yet been identified, and it is not probable that they will throw 
much light on the age of the plant-bearing deposit. The only direct palaeontological 
evidence of the place of the plant-bed in the sequence of strata is therefore afforded 
by the flora, the correlation of which will be discussed in what follows. A little 
south of the locality, on the summit of the high ridge on the south side of the 
stream, there was found in the sandstone a single ammonite which has been described 
by Prof. STOLLEY as Åncyloceras patagonicum n. sp. The stratigraphical relation of 
this fossil to the plant-bearing bed could not be ascertained by means of direct ob- 
servation, because of the interruption caused by the ravine of the little stream. TIt 
appears at least very probable, however, that the Ancyloceras-horizon is higher than 
that of the plants: at any rate, there can be no great difference in age. The An- 
cyloceras is stated by Prof. STOLLEY to indicate an Aptian age. 
From locality c the sandstones continue northwards up to the pass to Rio 
Carbön and the Rio Mayer valley. When our party, after having crossed the pass 
on our way from Rio Mayer, descended the valley of Rio Fösiles we found almost 
the whole upper part of the latter valley to be cut down through these sandstones. 
