208 Dr. Dawson — Geology of Prince Edward's Island. 



greater number from the South, side, perhaps because nearer to it. It 

 thus furnishes a remarkable illustration of the transport of travelled 

 stones at this period in different directions ; and in the comparative 

 absence of travelled stones in the lower Boulder-clay, it furnishes a 

 similar illustration of the homogeneous and untravelled character of 

 that deposit, in circumstances where the theory of floating ice serves 

 to account for it, at least as well as that of land-ice, and in my 

 judgment greatly better. 



The modern deposits observed were extensive beds of Peat, Oyster- 

 beds, or " mussel mud, " Dunes or sand-hills, Shore Eidges or 

 " shooting-dykes." 



Of the Peat deposits examined, the most important was that of 

 " Black bank " in Cascumpec Bay, which is reported by Dr. Harring- 

 ton as having an area of nearly three millions of square yards, and 

 an average depth of fifteen feet. It presents a steep front to4he 

 bay, the waters of which are now gradually removing it ; and a 

 little below low-water markx it has a layer of roots of trees which, 

 indicate a forest surface now under the level of the sea. Similar 

 evidences of modern subsidence were observed in other places. At 

 Gallas Point, for example, there is a layer of stumps five feet below 

 high-water mark. 



The common American oyster, Ostrea Virginiana and vai'. Borealis, 

 occurs abundantly on the coast, and large accumulations of its shells 

 with those of the mussel, Mytilus edulis, have been formed in some 

 of the bays and river estuaries. I was informed by Mr. W. H. 

 Pope, who has given much attention to this subject, that some of 

 these beds are fifteen feet or more in thickness. They consist of 

 dead shells, and in many places no living shells occur even at the 

 surface, the animals having been killed by the gradual approach of 

 the beds to the surface of the water, exposing them to the action of 

 the frost and ice and to invasion of sandy sediment. These beds of 

 dead oyster and mussel shells, with the mud filling the interstices, 

 constitute one of the most valuable deposits on the Island. Under 

 the name of " Mussel Mud," this material is taken up in great 

 quantity by ingenious dredging machines, worked from I'afts in 

 summer or from the ice in winter, and is applied as a manure to the 

 soil with the most excellent effects. It supplies lime and organic 

 matter, besides small quantities of phosphates and alkalies. 



The shells in these old beds are all of the long narrow form (0. 

 Virginiana), and Mr. Pope informs me the round form (0. borealis) 

 occurs at the surface in many places where the long narrow form is 

 found only a few inches below. It also appears that the modern 

 oysters procured in the upper parts of the rivers and on muddy 

 bottom tend to the long form, while those in more salt water and on 

 hard bottom are round. 



"Dunes," or mounds of drifted sand, are extensively developed 

 along the outer or north-west shore, where they extend in long lines 

 across the bays and parallel to the coast. In all they extend in length 

 about 45 miles, and are sometimes more than 40 feet high. Though 

 usually held together by the roots of coarse grasses, they are liable 



