Bd. III: 2) CONTRIBUTIONS TO THE GEOLOGY OF THE FALKLAND ISLANDS. 
1 1 
Die Niveaubestimmung war sehr leicht; denn alle Arten sind, wie mir scheint, 
solche, die in gleicher Häufigkeit auch in Argentinien und Bolivien sich wiederfin- 
den und dort das Unterdevon^ z. T. vielleicht noch das tiefere Mitteldevon, kenn- 
zeichnen. Das gilt insbesondere von Spirifer antarcticus MORRIS & SharpE, Lepto- 
coelia flabellites CoNRAD, sowie von ein paar Chonetes-Kritn {falklandica M. & Sh.). 
Auch was sonst vorhanden ist, Reste von Pliacops^ Cryphæjis (oder Dalinauites?) 
und Homalonotus. ein Bellerophon {Tropidocyclus) etc., spricht durchaus für Unter- 
devon so dass dies Alter als gesichert angesehen werden kann.» 
Diabase-dikes in the Devonian sandstone. 
As mentioned already in the introduction, »numerous basaltic dikes» were reported 
by Captain SuLlVAN as occurring in West Falkland. Nothing is published about 
the localities where these dikes were noticed or about their lithological character. 
At Fox Bay I found two small diabase-dikes traversing the Devonian sandstone. 
One of these dikes (No. i) crops out on the east side of the harbour close to the 
south of the entrance of the creek running from the harbour in a NE. -direction, the 
other (No. 2) was found on the east side of this creek. 
No. I is 1.5 m. broad, almost vertical and running E. — W. Much of the rock 
is deeply decomposed by weathering, which has advanced most rapidly along the 
joint-planes. Between them kernels of undecayed diabase, 0.02 — 0.4 m. in diameter, 
are very common. Thus the dike appears like a mass of brown ferruginous balls in 
a soft, entirely decomposed matrix. A microscopic slide from a fresh ball has been 
examined by Prof. Nordenskjold, who has kindly sent me the following note: 
»The rock is a fresh, beautiful olivine-diabase, that is characterized by the abundant 
occurrence of olivine in well developed, comparatively large crystals, all belonging to 
an older generation. At the outer surface and along irregular fissures the olivine crystals 
are seipentinized, an alternation that is not far advanced. Still the rock is not fresh 
enough to justify the name basalt. As porphyric individuals there occur also thin laths 
of a very basic plagioclase. Hardly any augite is noticed amongst the porphyric mine- 
rals, but the ground-mass consists of augite and plagioclase of common type. In the 
ground-mass no olivine was observed, but it is possible that originally small individuals 
of this mineral occurred, though they were afterwards completely decomposed.» 
The dike No. 2 is not exactly vertical, but dips steeply N.; its strike is equal 
to that of No. I, E. — W. As appears from fig. 6 this dike is double, divided by a 
flake of sedimentary rock, 0.25 m. broad, in a northern portion, 0.4 m. thick, and a 
southern, 0.6 m. in thickness. On the north side of the dike the slaty, micaceous, 
fossiliferous sandstone, that forms the surroundings of the dike, is gently inclined, 
on the south side it is horizontal. The narrow body of sedimentary rock between 
