G.M.DAWSON — GEOLOGY OF MIDDLETON ISLAND. L3] 



grounding ice, but this alone would appear to be scarcely sufficient to explain the 

 always broken appearance of the mollusks in the specimens actually to band. 



The distance from the border of the mainland (about 55 miles) would seem to in- 

 dicate that it represents a portion of the morainic deposits formed at the outer edge 

 or along the retreating front of that part of the continuation of the Cordilleran 

 glacier which is believed to have occupied the highlands of the corresponding part 

 of the Alaskan coast during the first and most important period of glaciation.* 



It will be noted that the island lies Opposite an extensive indentation in the 

 general coast line, marked by Prince William sound and also by the Copper River 

 valley, and it is therefore possible that the corresponding portion of the great 

 glacier here stretched further seaward than elsewhere. The water between the 

 mainland coast and Middleton island is not very deep, varying, according to the 

 few soundings shown on the chart, from 30 to 50 fathoms. It is therefore quite 

 probable that a glacier-sheet moving outward from the land may still have borne 

 upon the sea-bed with sufficient weight to produce the effects above alluded to, 

 even were the relative elevations of sea and land the same as those of to-day. 

 There is, however, so much reason to believe that very extensive changes in levels 

 have occurred in the region during and subsequent to the glacial period, that it is 

 not safe to assume that the relative levels were identical with those now existing. 

 It is reasonably certain that the island, composed of such relatively soft material, 

 and exposed as it is with few protecting beaches to the full force of denudation 

 exerted by a stormy ocean, has not for any very protracted period, from a geologic 

 point of view, stood at its present level. Mr Macoun's description of the western 

 side of the island in fact distinctly indicates the existence there of a well-marked 

 terrace, cut back at a height of al tout 100 feet above the present sea-level. Whether 

 this actually represents, in a modified form, that pause in elevation which the 

 coast further south seems to have experienced during the closing events of the 

 glacial period (there at an elevation of about 200 feet) t it is difficult to say ; but 

 it indicates, with scarcely any doubt, one stage in that general and last process of 

 elevation. The unoxidized character of the bowlder-clay itself seems to show that 

 it can never for a very prolonged period have been subjected to subaerial agencies. 



In Dr Dall's observations on Middleton island, already quoted, the following 



statements are in conclusion made : 



"Below the sea-level some of the rock appeared to be quartzite in place an.d very hard. What- 

 ever its nature.it extends in reels and shoals to a distance of sevei-al miles from the island in 

 different directions. No fossils were found in the clay stone, but from its character it was suspected 

 to be post-Miocene and possibly Pliocene " J 



Respecting the existence of a quartzite basis of the island, Dr I >all writes doubt- 

 fully as above, while Mr Macoun did not note any such underlying rock in follow- 

 ing the shores. It would appear to be very probable that the surrounding reefs 

 or shoals are merely the higher parts of a plane of marine denudation or banks 

 thrown up upon such a plane, which now surrounds this rapidly diminishing 

 island and corresponds with its original size under the existing relative levels of 

 sea and land in the region. As to the age of the material composing the island 

 itself there seems to be no room for doubt that this is Pleistocene and referable to 

 the Glacial Period. 



* Later Physiographical Geology of the Rocky Mountain Region, etc : Trans. Royal Soc. Canada, 

 vol. viii, see. iv, map I. 

 f Ibid, p. 54. 

 I Op. supra cit., p. 200. 



