CHAPTER III. 



CORRELATIONS OP STRATIGRAPHY, LITHOLOGIOAL CON- 

 STITUTION OF STRATA, AND OROGRAPHY. 



For a number of years I Lave paid special attention to the subject, 

 to what extent do the physical constitution of strata and their struc- 

 tural condition influence the orographic features of any given region? 



Both in Europe and in the Uuited States 1 have collected data with 

 a view to presenting at some time a synopsis of the observations made, 

 and to iuduce from such synopsis the probable results that would be 

 derived from given lithological and structural character. Upon examina- 

 tion, I find that to a great measure the effects from similar or identical 

 causes correspond very well. A recognition of this fact has direct bear- 

 ing upon the recognition of minor details in mountain ranges and groups. 

 It will facilitate the study of physical geography and frequently afford 

 a clew to the explanation of orographic forms that otherwise might be 

 difficult. 



It does not lie within the province of this paper to discuss either the 

 causes by which abnormal positions of strata are produced, or the in- 

 fluence that the action of any plutonic or volcanic activity may have had 

 upon strata or groups. We begin with a comparison of the result pro- 

 duced upon strata or lithological groups by atmospheric and other 

 eroding agents, after they have assumed a rigid stability. Noticing 

 the varying forms, the resultants of similar or identical influences to 

 which each and every one of them has been or is being subjected, wo 

 •will perceive at a glance the value of a system of reasoning based upon 

 inductive principles. We have analogous, if not absolutely the same, 

 agents producing a multiplicity of results, and by^ the study of these 

 results can arrive at some conclusion regarding the physical structure 

 and constitution of the media exhibiting them. Indirectly, again, we 

 can, by combining empirical knowledge with the results presented, 

 argue an inference as to the mineralogical and chemical aggregates and 

 constituents composing the forms under consideration. 



An application of the knowledge we may derive from the study of 

 cause and effect, the former being almost constant in this instance, will 

 enable us to draw comparatively correct conclusions as regards the 

 general character of the material in which such effect is observed. This 

 is of importance in geological and geognostical work. When in the 

 lield, the first intimation of the age or relativ^e position of a stratum 

 may frequently be furnished by some peculiar feature of weathering. 

 If we have for examination before us a circumscribed area, in a portion 

 of which the age and character of the various geognostic horizons have 

 been definitely determined, a study of the exterior physical appearance 

 may often greatly facilitate the recognition of any one of the determined 

 horizons at an even distant locality. No doubt this method is employed 

 almost instinctively by most or all geologists who have occasion to ex- 

 amine regions comparatively or entirely unknown. It is no6 my pur- 

 pose, however, to write a "guide for field geologists j" so this instuuce 

 of direct practical application may suffice. 



During four years I have had the opportunity of studying geology in 

 Colorado and a portion of its contiguous Territories. The survey of this 

 State is now completed, and I send forth the subjoined notes with espe- 

 cial reference to what I have observed in that State. Few regions, per- 

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