﻿N. S. Shaler — Geology of Cobscook Bay District, Me. 41 



ward of Moose or Eastport Island. This series of ash beds 

 covers a considerable area ; in this part of the district the beds 

 have the average dip of the rocks in the region and though 

 their upper and' lower contacts with the series of fossil-bearing 

 beds is not well determined there can be little doubt that they 

 are a constituent member of the Paleozoic section. 



The best exhibition of these volcanic ash deposits is at Mc- 

 Master's Island, a small peninsula about midway of the western 

 side of Moose Island. This little island, of only a few acres in 

 area, is almost altogether composed of beds of a fragmental 

 nature. At first sight and from a little distance these beds 

 closely resemble coarse gravel and conglomerates ; but, on 

 closer inspection they are seen to be made of fragments mostly 

 angular, which though lying in regular beds have their major 

 axis at all angles to the horizon ; moreover, the larger bits which 

 exceed a foot in diameter have evidently fallen with a certain 

 violence into their places as is shown by their crushed form. 

 All these fragments seem to be composed of volcanic or crys- 

 talline rocks, among them are many bits of pumiceous material 

 which is rarely so vesicular as to deserve the name of pumice 

 but distinctly related to that class of volcanic products. The 

 evidence derived from the section at McMaster's Island and 

 from the other exposures in the vicinity show that the set of 

 volcanic beds exceeds five hundred feet in thickness and 

 may be far thicker. It is likely from the width of its glacial 

 trail in the region to the southward that this series of rocks 

 occupies a good deal of the space of the channel known as 

 Moose river. 



2. 



West side of Moose Island near N. end. 

 A, greenish ash beds. B, green siliceous flags. C, contact — obscure — appar- 

 ently overlie. D, red felsite dyke 20 ft. wide. 



The evidence goes'* to show that this series of McMaster's 

 Island was formed some time before the Hamilton period. It 

 seems clearly overlaid by the extensive series of the Shack- 

 ford Head group and the equivalent beds of the Princes Cove 

 section, while its base appears to rest upon the rocks which are 

 probably of Silurian age. 



There are some other points in the Cobscook section whose 

 rocks of the same general structure as those of McMaster's 

 Island are exhibited. The other exposures may or may not be 

 of the same age as those before described. They probably rep- 

 resent several stages of volcanic eruption at various periods in 

 the Silurian age. 



