MEXICAN BOUNDARY LINE. 117 



assume more importance^ and extend into long belts. In the Limpia range these rocks present 

 the character of a mountain chain, having an elevation of 6,000 feet, and extending several 

 hundred miles north and south. The specimens from this range present the characters of erup- 

 tive and metamorphic rocks. Notwithstanding the syenitic texture of some of the beds, they 

 have still a modern aspect. The different minerals are quite distinct from each other, not 

 blended and imbedded as in the older metamorphic rocks, and their mode of aggregation is also 

 unlike. In addition to this, the occurrence of igneous products of very modern age, which are 

 intimately associated with these rocks, and apparently prevail in great quantity, induce us to 

 regard all these as belonging to a system of eruption and of elevation of very modern date. 



We may, however, inquire what other evidences, if any, we have in the surrounding rocks 

 as to the age of these igneous mountain ranges. The great table-land formed of the cretaceous 

 rocks has on its eastern margin an elevation of not far from 1 ,000 feet. The surface of the 

 country gradually rises to the westward,, and near its junction with the igneous rocks of the 

 Limpia range they have an elevation of 3,000 or more feet. On approaching the range, also, 

 we find these beds of cretaceous age dipping at a high angle in various directions, showing 

 great disturbance of the beds, apparently due to the elevation of intruded igneous masses. The 

 beds of cretaceous rock have in some instances been indurated, and otherwise affected by the 

 proximity or contact of igneous masses. 



We have, therefore, not only evidence of the general elevation of the country towards the 

 great central range in the inclination of these beds, but we have the positive evidence of local 

 disturbance and change due to the intrusion of these igneous masses which form isolated 

 points or mountain chains. 



Beyond the Limpia range, in the neighborhood of El Paso, we have cretaceous rocks, con- 

 taining numerous fossils. These beds rest upon carboniferous limestone, and all have a westerly 

 dip — the carboniferous strata dipping at a much higher angle than the cretaceous. The rocks 

 of both periods are complicated with volcanic and other igneous rocks ; and in some instances 

 the latter have been protruded beneath the cretaceous beds, and rest upon the carboniferous 

 limestone, which is but partially altered. The cretaceous beds of this locality are about 4,000 

 feet above tide water. 



Still farther west, in the vicinity of Corrilitas, cretaceous beds occur in connexion with par- 

 tially altered limestones and igneous rocks, having an elevation of nearly 5,000 feet above the 

 level of the sea. This is the most westerly point at which any cretaceous fossils have been 

 found on the line of this expedition. 



The occurrence of cretaceous deposits in this region is of much interest when taken in con- 

 nexion with similar discoveries further to the northward. Captain Fremont, in his explorations 

 of 1843 and 1844, brought cretaceous fossils from the eastern slope of the Rocky moimtains, 

 Smoky Hill Fork of the Kansas river, in latitude 39°, longitude 105°. In the explorations of 

 1846 and 184T, Lieutenant Abert collected specimens of the same species of cretaceous fossils 

 (Inoceramus mytiloides, = I. problematicus ,) at Poblazon, on the western slope of the Rocky 

 Mountains, in latitude 35° 13', longitude 10T° 0' 2".* 



' Professor Bailey, wlio identified the fossils in Lieutenant Abert's collection, makes the following remarks : " The fossils 

 from Poblazon consist of gigantic Ilippurites, casts from the cells of several species of Ammonites, valves of Inoceramus, 

 identical with a species figured in Fremont's Report, pi. IV, fig. 3,t casts of small univalves and bivalves too imperfect for 



t Inoceramui {mytiloidts) probUmaticiu. 



