KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 51. N:O 3. 19 
Somewhat below the pass but a considerable way up in the sandstones large am- 
monites, similar to those mentioned from locality c, were noted. On about the same 
level were also found a few plant-impressions, viz., Sphenopteris psilotoides (SToKES 
& WEBB) WARD [= Onychiopsis Mantelli (BRGN.) NaArtH.], Sphenopteris cf. nakton- 
gensis YABE, and probably fragments of dicotyledonous leaves. Sphenopteris psiloto- 
ides was found a little higher up than the other fossils, but it is not certain that 
it actually represents a stratigraphically higher horizon. At any rate there can be 
no considerable difference in age. The thick beds of sandstones which underlie this 
plant-bearing stratum compels one to the idea that it occupies a fairly high horizon 
in division 6. There can hardly be any doubt that these plants are younger than 
those of the locality c, though the amount of the difference cannot be ascertained. 
This question will receive fuller attention in the discussion of the flora. 
As has already been stated, the sandstones of division 6 continue on the north 
side of the pass, Rio Carbön being for the greater part of its course cut down in 
these rocks. Further north, at the junction of Rio Carböén and Rio Mayer, the sand- 
stones are found to rest upon black slates similar to those at Bahia de la Lancha 
and evidently representing division 5. These slates continue to the west up the courses 
of Rio Mayer and Rio Pinto. They become more and more disturbed towards the 
Cordillera, being strongly folded and cut through by eruptives, essexite according to 
QUENSEL (1911, p. 69). These black slates are the Mayer River beds of HATCHER 
(1897, p. 328; 1900, p. 88), who has studied them in many places in the Rio Mayer 
Basin. Unfortunately he has found no determinable fossils, and the few invertebrates 
collected by myself in this district are not likely to throw much light on the question 
of the geological age of these beds. There can be no doubt, however, that the 
Mayer River beds are identical with the similar black slates which underlie, on the 
south side of the pass, the same sandstone series. Thanks to Prof. STOLLEY'S 
determination of the few Cephalopoda from Bahia de la Lancha it is thus possible 
to arrive at a fairly accurate determination of the age of this wide-spread and im- 
portant formation, the oldest fossiliferous horizon in this portion of Patagonia. 
It may be mentioned here that I have found much further north, in the valley 
of Rio Aysen, some black caleareous slates of very much the same aspect as those 
of division 5 in the Lago San Martin district. The fossils collected here have not 
yet been determined, but according to a letter from Prof. STOLLEY, some fragments 
of a Belemnite indicate Upper Jurassic. It is to be hoped that the fossils will prove 
whether the deposit at Aysen is homotaxial with some portion of the Mayer River 
beds and division 35 in the Lago San Martin district — for my own part I think 
this is very probable. The Aysen deposit, which is of great interest also because 
of its occurrence in undisturbed horizontal beds far towards the centre of the Cordil- 
lera will be more fully treated in another paper which is to be published as soon as 
all the fossils have been determined. 
