IoFor several years past, while travelling professionally in the Rocky Mountain area in the United States and Mexico, I have been profoundly impressed by the acute crenulation of mountain ranges which spring from the elevated plateaus of the Rockies. I use the term "crenulation" advisedly. Folding into anticlines and synclines is a term altogether too mild to suit the facts. The sudden and acute changes of dip and strike, changes from o ¸ to 9 ø0 or more in dip (in the case of "more" an overthrow fold is implied) seem, at first sight, to call for profound earth movements with corresponding faults and fissures of commensurate depth.In considering this plausible explanation one is immediately confronted with a paradoxical if not a contradictory negative phenomenon. In profound rupturing or faulting, one would naturally expect eruptives in the form of bosses, intrusives and dikes, either one or all; while the contrary is in many cases true, all forms of eruptives are strikingly conspicuous by their absence.The interpretation of the above has a decided economic value, but more than this, even, the calculation of the depth of a given fissure often becomes of prime importance from the standpoint of the miner.Mineral deposits of all kinds are found in fissures (so-called veins) in blankets or their Mexican equivalents, "Mantos," in lenses and "shoots," "chimneys" and "blowouts." These terms have no exact and distinctive meaning but depend, for their application, wholly upon the interpretation of the individual. The reason for this confusion is not far to seek. It appears to me that pains have not been taken to reason out carefully the basic facts of these phenomena. 42I
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