104 GEOLOGY AND PALEONTOLOGY. 



portation) is apparently identical with specimens found in the immense drift deposits far to the 

 northward, in Nebraska and at other places ; and although quite possible that both the trilobite 

 and the coral have been derived from the extreme northern exposures of the older rocks, I am 

 nevertheless prepared to exjaect that these formations will be found nearly coextensive with the 

 carboniferous limestones. 



Although there are among the collections of the Boundary Survey several specimens which 

 appear referable to a position below the carboniferous limestones, and in the later collections 

 some Silurian or devonian corals, yet in the absence of other information than that furnished 

 by the specimens, which do not bear evidence of having been freshly broken from the rocks, I 

 do not feel warranted in drawing any general conclusions. This subject is one of the greatest 

 interest for future explorers in that region. 



The specimens of igneous and metamorphic rocks from the eastern and central portions of the 

 route travelled are all unlike those so well known in the eastern part of the United States ; and 

 it is not until we approach the range of the Cordilleras that we find specimens bearing all the 

 lithological characters and associations of the metamorphic rocks of the Appalachian chain. 



We are constrained to believe, not only from the evidences of this collection, but from others 

 previously examined, that the metamorphic rocks of this intermediate region are to a great 

 extent newer than those of Silurian and Devonian age, which we know to be the age of a large 

 part of the metamorphic portions of the Appalachian chain. This conclusion, or we may say 

 suggestion, is deduced from the differences in lithological character, as well as from the fact, 

 already stated, that the upper carboniferous limestone is the most conspicuous unaltered rock 

 of the region, while in some places this rock itself, as well as strata of more recent date, 

 appear to pass into a metamorphic condition. 



In regard to this carboniferous limestone, it should also be borne in mind that it is not the 

 carboniferous limestone of the Mississippi valley which attains this force farther west, but a 

 limestone high in the coal series, and which has become thus developed, while we are yet 

 ignorant of the existence of the lower carboniferous limestone in that part of the country. 



The collections of the Boundary Survey, when compared with those made in traversing the 

 country along lines farther to the northward, present a great similarity of aspect and litho- 

 logical character. The cretaceous belt, bordering the metamorphic and igneous region which 

 succeeds in turn, and in the midst of which are large areas of limestone just mentioned, and also 

 some smaller areas of cretaceous rocks, which have now been traced quite to the central part of 

 the great basin to the west of the first mountain chain, and have more recently been 

 determined in California. 



We have the means of knowing, therefore, the general geological structure of the country, 

 from near the northern limits of the United States to Mexico. 



The first part of my report, constituting the observations upon the specimens collected, and 

 the general results regarding the geographical distribution of the formations along the line of 

 the Boundary Survey, are essentially the same that I transmitted to you in 1854 ; but which, 

 in consequence of the change in the boundary line making a re-survey necessary, were not 

 published at that time. 



I have since reviewed the whole collection, with all the additional information derived from 



