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National Geographic : 1892 Mar 21
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H. F. Reid-Studies of Muir Glacier. tudes and to dip in several directions, generally at high angles. They seem to follow the fissure systems. Their width is com monly but a few feet, but some were found twenty feet and over. They keep their width with great persistency for considerable distances. No surface flows were found. To the eye all these dike rocks strongly resemble one another.. They have a nearly black or dark greenish color, a sandy appearance on weathered surfaces, and are generally distinctly porphyritic in structure, with considerable variation in the size of the porphyritic crystals. The specimens I collected from these small dikes and sent to Dr Williams he classifies without exception as diabases. From a somewhat larger and much decomposed dike on the ridge east of Dirt glacier valley I. sent him a specimen which he describes as micropegmatite. What relation this dike bears to the diabase dikes could not be determined, and I found no others like it, though the loose pieces on the moraines show that they occur to the north. With this one exception, all the dikes seen presented a very uniform appearance, save for such slight differences of texture as depend on slight variations in the rate of cooling. No evidence was found of successive outflows of varying charac ter, though that may be furnished by the exploration of a wider area. The dike rock occurs in blocks of varying shapes and sizes. How much of this is due to fissuring and how much to contraction of cooling I cannot say. That a certain amount of dislocation has occurred since their formation is evinced by the fact that occasionally they are somewhat faulted. But their appearance in the field shows that they have suffered but little dynamically compared with the enclosing rocks. No direct evidence was forthcoming, bearing on the age of these later eruptives; all that can be stated with certainty being that they are the youngest rocks hereabouts and are contemporary with a great disturbance of the region. Their appearance is very similar to that of certain eruptives of Tertiary age occurring in the Cordilleras further southward. The earlier eruptives indicate a certain amount of disturbance of Paleozoic or Mesozoic date in the region. At a later date further and greater movements took place, the rocks were upturned, faulted, and fissured, and certain of the fissures were penetrated by lavas of Tertiary habit.* * These later eruptions probably took place at the time of the upheaval of the St. Elias range. See page 24 [H. F. R.].
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