MAitvKE.] GEOLOGY ROCKS EAST OF FEOiNT RANGE. 93 



C H A P T E R IL 



THE SEDIMENTARY ROCKS EAST OF THE FRONT RANGE. 



Before presenting in detail the isolated facts which together make up 

 our knowledge of the sedimentary rocks east of the mountains, and in 

 order that these facts may be more readily understood and their rela- 

 tions rendered apparent, it is desirable first to clearly understand cer- 

 tain general geological features which underlie the whole country. These, 

 though familiar to most geologists, may be less so to the general reader' 

 and I will endeavor to present them as simply as possible. 



Attention has already been called to the abruptness with which the 

 mountains rise from the plains. The two types of topography which 

 are separated by this sharp line of demarkatiou are not more wholly 

 distinct than are the two types of geology which it also separates. 



The rocks of which the mountains are built, and which will be more 

 fully treated in the following chapter, are granite, gneiss, schist, and al- 

 lied rocks, which, in speaking of them as a class, it will be convenient 

 to call ArcJicecm, or metamorphic* 



These metamorphic rocks are hard and crystalline in character, and, 

 occurring .in large and comparatively irregular masses, present surface 

 forms which are peculiar and very characteristic, so that one can sel- 

 dom fail to recognize them even from a distance. 



Beneath the plains, however, there is found a class of rocks which 

 possesses characteristics exactly the opposite of those just mentioned. 

 They are neither so hard, nor are they crystalline 5 and instead of occur- 

 ring in large and irregular masses, they lie in broad, flat sheets or strata, 

 resting one above another, and stretching unbroken for miles in all di- 

 rections. From the Mississippi and Missouri westward to the mount- 

 ains stretches this great series of rockSj composed of layers of varying 

 thickness of sandstones, limestones, shales, slates, and clays, which re- 

 main much as they were when first laid down one after another in the 

 bottom of the vast ocean which once existed here. Since this ocean 

 was gradually drained off, the ceaseless action of the rains and rivers has 

 in places removed thousands of feet of these rocks, exposing beds which 

 were once deeply buried, and in which we can occasionally find the re- 

 mains of the shell-covered beings which still earlier lived and died upon 

 the ocean-bottom, or the skeletons of the animals and plants which peo- 

 pled the surrounding shores, and were swept by the ancient rivers out 

 into the sea to be buried with the then forming sediments. It is but 

 natural, therefore, that the surface characteristics of the great plains 

 and rugged mountains should differ as they do. 



THE MOUNTAIN BORDER REGION. 



But let us examine the line along which these two opposite classes of 

 rocks maybe supposed to join. This line, we have already seen, trends 



"* It has been found convenient, or rather necessary, for geologists to give definite 

 names to certaitf groups of rocks. The accepted names of the more general diyisions, 

 in the order of their superposition or age, commencing with the oldest and proceeding to 

 the most recent, are here given, those in small capitals occurring in the region uuiler 

 discussion : 



1. Ar.cniEAN — most ancient. 6. Jueassic. 



2. Silurian. 7. Cretaceous. 



3. Devonian. C Pliocene, 



4. Carhomferous. 8. Tertiary^ Miocene, 



5. Trias.sic. I Eocene. 



9. Fost-Tehtiaky and recent. 



