Structure of Bocks and Mountains. 57 



great difficulty in inaking determinations of dijj on account of 

 the obliteration of the bedding planes ; Ijut it is clear that a 

 considerable amount of faulting took place prior to the forma- 

 tion of the dikes. The strike of the sedimentary rocks is about 

 N. 50° W. (N. 80° W. magnetic). The dips are generally at high 

 angles toward the south, but are extremely variable. Short 

 monoclinals involving a considerable change of dip are not un- 

 common. Small anticlinals and synclinals occasionally occur, 

 but are of minor importance, soon giving way to the prevailing 

 southerly clip. These frequent variations in dip are well shown 

 along the eastern shore of Headland island. A considerable 

 fault is also shown about midway along this shore, clearly indi- 

 cated by the difference of dip on the two sides ; but the simi- 

 larity of the rocks on both sides renders any determination of the 

 amount of throw impossible. Sufficient country was not cov- 

 ered to at all clearly exhibit the type of mountain structure 

 obtaining here. The dip is pretty persistently away from the 

 diorite peaks, which were the most northerly ones reached. 

 What lies beyond them no man knows. The work done makes 

 it probable that we have here a tilted and raised hlook accom- 

 panied by faulting. 



Sedimentary Rocks. 



Two great series of sedimentary rocks are exposed in the 

 Glacier bay region, one of argillite, the other of limestone ; both 

 are of great thickness. The limestone is the younger. The 

 contact between the two is well shown on the eastern shore of 

 the bay, has an extremely steep southerly dip, and reaches tide- 

 water about eight miles south of the present front of Muir 

 glacier. 



The Argillite. — The mountains adjoining Muir inlet and the 

 upper northeastern shore of Glacier bay and those surrounding 

 the eastern part of Muir glacier amphitheater are entirely com- 

 posed of slaty rocks (see geologic map, plate 16). The thickness 

 of these argillites I was unable to determine, but it clearly reaches 

 several thousand feet. They present three main phases : 



1 . Very hard, fine-grained, gray argillo-siliceous bands, of some- 

 Avhat varying shades, occasionally approaching quartzite in 

 character. 



2. Equally abundant with the last, and with them making up 



