(32 //. F. Held — 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 hut 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 re.semlile one another. They have a nearly 

 black or dark greenish color, a sandy aj^pearance on weathered 

 surfaces, and are generally distinctly j)orphyritic 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 sa}^ That a certain amount of 

 dislocation has occurred since their formation is evinced by the 

 fact that occasionally the}^ 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 

 Avith a great disturbance of the region. Their appearance is very 

 similar to that of certain eruptives of Tertiary age occurring in 

 the Cordilleras further south Avard. The earlier eruj^tives 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 uj)turned, faulted, and fissured, and certain 

 of the fissures Avere penetrated by lavas of Tertiary habit.* 



"••' These later eruptions probably took place at the time of the upheaval 

 (jf the St. Elias range. See page 24 [H. F. R.]. 



