298 Tavels of a Naturalist in the Alps. 
spar are also discernible in the aggregate in certain places. By de- 
grees, as the formation proceeds, it becomes a homogeneous, siliceous 
slate, which in many places is also vesicular. The quartz now sud- 
denly disappears, and the particles of feldspar only remain. ‘The 
general mass becomes a blackish, slaty clay with a shining shistose 
surface and a coarse cross fracture, in which individuals of white feld- 
spar are visible. An increase of lime now commences, and with it 
reappear the petrefactions, such as Belemnites, Ammonites, Mytili- 
tes, Terebratulites, &c. This porphyritic greywacke-slate may be 
regarded as the commencement of the lias, while the pisiform iron- 
stone forms the conclusion of the Alpine limestone ; consequently 
only the granitic sandstone can be considered as constituting the in- 
termediate or interposed formation. 
As the lime obtains the ascendancy in the porphyritic slate, the 
feldspar disappears, and the earthy granular fracture changes into 
a pisiform or concentric granular structure ; the Belemnites and Am- 
monites disappear, while the Terebratulites, Mytilites, &c. still re- 
main, but only in the inferior layers. It is of immense thickness, 
and is characterized as follows: The color is black; when the lime 
predominates, (in consequence of which the layers become thicker,) 
its structure is oolitic and frequently crystalline ; but when alumina 
prevails, (which renders it shistose,) the cross fracture becomes even 
and sometimes splintery ; and finally, when silica is superadded in 
large proportion to both, the texture becomes coarse, like that of 
greywacke. No formation is so protean as this; at one time, it pre- 
sents us with a series of wackes, at another with marls, gryphite-lime- 
stone, &c. Indeed, Huet is of opinion from a close examination of 
the Jura and the Alps that these, as members of one and the same 
leading formation, occupy under a multiform alternation with each 
other, nearly the entire space between the Alpine and Jura limestone. 
It appears therefore, that with a single interposed formation, the 
Alps contain but two leading limestone formations, above which oc- 
curs, in particular spots, a third, the chalk with Jura limestone. 
It remains, in the enumeration of the Alpine series, to speak of 
the secondary granite, whose existence the author seems to have es- 
tablished on very satisfactory grounds, notwithstanding the opinion 
of some geologists that what he has taken for granite is only a variety 
of sandstone. The observations of Exiz pe Beaumont in the 
Montagnes d ’ Oisans* fully confirm the ideas advanced by Huet. 
* This geologist found granite cutting through, and resting upon, deposits be- 
longing to the oolite series. 
