604 TIERRA DEL FUEGO. 
on an inclined dike, dipped at an angle of seventy degrees, while those 
on the other side dipped thirty degrees. 
Fossils —Only a single species of fossil was observed in this forma- 
tion. It was found on the shores of Nassau Bay, about half way from 
Orange Harbour to the head of the bay, and occurred in a compact 
argillaceous shale, where the rock was passing to an argillaceous sand- 
stone. The fossil is represented on Plate 15, figure 1, and is allied to 
the Belemnites, constituting the new genus Helicerus.* 
The specimens were found only in a single layer of the rock; they 
were quite thickly distributed, fifteen or twenty occurring in a slab a 
foot square. 
Concluding Remarks.—F rom the character of this formation, it may 
be inferred that it is of marine origin, and that a large part of the 
material has been derived from the igneous rocks of the region, or 
others allied. Even the sand-rocks appear to be made of the same 
material. The blue shales have less resemblance to rocks from such 
a source. 
The occurrence of a Belemnite in the rock gives us some ground 
for arranging the beds with the Oolitic series, and probably in the 
upper part of this series; yet the fossil is so peculiar, that the conclu- 
sion cannot receive our full confidence until corroborated by further 
investigation. 
Igneous Rocks. 
The summits of the hills in general consist of the outcropping 
igneous rocks of the region. Over the hills between Orange Harbour 
and the head of Nassau Bay, the sedimentary rocks rarely come to 
the surface, owing to the covering of soil and vegetation ; and igneous 
rocks are therefore the only kind which comes in view. 
These igneous rocks are various in character. The most abundant 
variety is a greenish-black greenstone of fine texture, breaking with 
little lustre, though tough. In some places the colour is grayish-black. 
Minute crystals of feldspar are often thickly disseminated, and large 
crystals of hornblende are sparsely distributed through some varieties. 
Certain dikes abound in veins of chalcedony, with occasional geodes 
of quartz crystals ; and these veins when thin have often a red colour. 
Much of the greenstone sensibly affects the magnetic needle. 
* See Appendix. 
