PALEOZOIC. 53 



veins. The curving form of the smaller feldspar veins would also suggest 

 an intermediate compression, during which the granite became sufficiently 

 viscous to admit of some movement within its mass without producing 

 fracture, for it is fair to assume that these A^eins are the filhng of a crack 

 along a fracture plane, and therefore originally more or less straight. 



PALEOZOIC. 



The sedimentar}' deposits later than the Archean which are found in 

 this region belong, with the exception of certain very recent beds, to the 

 Paleozoic system. In the multitudinous sections afforded by the expos- 

 ures along the cliffs of amphitheaters and the walls of canons remarkable 

 uniformity in the physical characteristics of these beds is observed. Prac- 

 tically the same bed, a fine-grained conglomerate, is, with a single excep- 

 tion, found in contact with the underlying Archean wherever the contact is 

 exposed and no non-conformity of stratification or other evidence of a phys- 

 ical break exists. 



In determining the geological age of the diff'erent strata included in 

 these series two difiiculties are met at the outset : first, the rarity of fossil 

 remains in the beds, due probably to their relatively metamorphosed and 

 altered condition; second, the absence of any sy.stematic description of the 

 Paleozoic horizons of the Rocky ]\lountain region, to be found in the pub- 

 lished works of other geologists. The voluminous reports of the Hayden 

 Survey contain, it is true, many local sections of sedimentary rocks and 

 fi'equent surmises as to their age, but as yet, unfortunatel)', a systematic 

 summary which shall correlate the material thus gathered by many differ- 

 ent individuals into a harmonious whole, and sift out that which is to be 

 considered fact from that which is only surmise, is wanting. 



It has long been the opinion of the writer, and one which is confirmed 

 by later geological investigations, that it is impracticable to determine by 

 similarit}^ of molluscan fauna alone the coiTespondence of beds and forma- 

 tions in regions so widel}^ separated as are the Rocky Mountains, where as 

 yet meager data have been gathered, and the Eastern States, where pale- 

 ontological horizons are firndy e.stablished. In Paleozoic times these regions 

 were practically two distinct continents, and the conditions of life must have 

 varied considerably. Until, therefore, the sequence of development and of 



