l')4 JJii AiiKricii II (fi<il(Kj/sf. Sf|)(i'iniHT, isiii 
uiid are exposed either by the removal of the latter through erosion, or 
being upturned in the two trreat moimtain systems, which limit the 
region — the Ouachita on the north, and the basin ranges of the Trans- 
Pecos region and northern Mexico on the west. 
;5. The above mentioned mountain systems, the first of which the 
Ouachita system of Arkansas and Indian Territory is older than the 
plains of the coastal system, and against which they were laid down ;and 
second the Basin mountains which are composed of the uplifted, folded 
and crumpled southward edges of the earlier of these plains ; i. e. 
those founded on rocks of Cretaceous age. 
4. Plains of later age than the mountain foldings which were laid down 
against these newer mountains, and these include the Llano Estacado, 
and the later formations of the coastal series ; and lacustral basins which 
were laid down between the mountains and in valleys of the plains like 
the dry lake beds of the latter region. 
Many of tliese features have been described in previous papers, 
especially those of the eastern half of the Texas region, and in the 
present paper. I wish to contrilnite a few facts concerning the 
Texas-Mexican extension of the Basin region. 
Among the most conspicuous of the basins are the lakes La- 
honton and Bonneville, numerous unnamed Itasins in Arizona and 
in New Mexico: the Mesilla valley, the Franklin-TIueco valle}', 
and Kl Jornado del Muerto in New Mexico; the valley of the Salt 
Lakes, the Eagle Flats, and the Toyah- Pecos Ijasin in Texas; and 
the basin of Presidio Del Norte, plains of Chihuahua. El Bol- 
ion de Mapimi; the plains of Lago Aqua Verde. Baroteran Bar- 
real del Jnnco. A'alle TTundido. Yalle Labago. (^ayote and nu- 
merous others. 
It should be borne; in niiiid thai these phtins covi-r numerous 
areas, and occupy most of the region, the niouiitaiu.s l»eing far 
secondary to them in extent and aveal importance. 
(iKo(iK.\lMllC KXTKNT OF THE K.XSIN RK(!10.\. 
The topography of the whole of the United States, and north- 
ern Mexico, soutii of the 33° of latitude between the Sierras of 
California, and the Pecos-Lower Rio Grande, may be defined as 
a series of vast plains or ancient base-levels studded at remote 
intervals by mountain blocks sometimes isolated, sometimes in 
groups or chains. These mountains lielong to the style defined 
by Russell as composed of stratified sedimentary beds which have 
l)een broken by profound fractures, and upheaved as great moun- 
tain blocks. Surrounding tliese mountain l)locks (and composed 
