142 M. de Buch on the 
western part of America, the chalk strata reach to 50° of 
north latitude, that is to say, 10° higher than the east coast. 
Their development, also, in this place, is greater than that 
of any other formation known on the surface of the globe. 
Captain Fremont has seen, in the neighbourhood of the River 
Plate, chalk strata with scattered specimens of Inoceramus 
Gripsii. They have likewise been found in the Arkansas, 
and as far as Santa Fe, in New Mexico, near Monterey and 
Laredo, by Dr Vislicenus, as appears from reports made to 
the Congress of Washington in 1848. In the Rocky Moun- 
tains and their prolongations to the east of Santa Fe, in New 
Mexico, this chalk-sea appears completely interrupted. 
Captain Fremont could discover no traces of it towards the 
River Columbia, nor towards the River Humboldt, in the 
singular great basin which extends as far as the Western 
Ocean. 
It is further observable, that here this vast formation of 
chalk consists only of the superior beds. According to very 
extensive and correct researches, Sir Charles Lyell is of 
opinion, that throughout the whole of North America, only 
strata of chalk are met with, which extend from the chalk of 
Maestrich to the gault; and M. Ferdinand Romer, in conse- 
quence of his accurate researches in the Texas, goes so far 
as to affirm that all the formations of that country, which is 
already sufficiently remote from the Atlantic coast, ought to 
be considered only as superior beds, which do not even belong 
to the gault. 
These surprising phenomena are, however, limited to 
North America. Even in Mexico we see deeper beds make 
their appearance. 
M. Galeotti has brought back from Tehuacan, towards the 
limits of the province of Oaxaca, trigonize, which he has de- 
scribed as the Trigonia plicatocostata (Bulletin de Bruxelles, 
iii, No. 10). This trigonia belongs to the subdivision of 
the Trigonie scabrw of Agassiz, and differs little from the 
Trigonia aliformis, Sow. It is characteristic for the middle 
chalk, chloriteous chalk, and even the gault. According to 
M. Galeotti, it is found in the middle of the great cordillera of 
Anahuac, 12 leagues to the north-west of Tehuacan, in such 
