THICKNESS AND POSITION. 321 



slate to appear if it lias not entirely thinned out before reaching that point of latitude. 

 Between the springs and Swanton Falls, the rocks in place are so much obscured by the 

 overlying alluvium, that it is difficult to know what kind of ledges occur beneath. 



But while it seems strange that so little of the Hudson River slate appears between 

 Swanton Falls and the Canada line, we can find reason for its disappearance in the fact 

 that there has been an elevation of an older rock at Highgate Springs, which also must 

 have elevated the slates. Because denudation has been great at this place, the easily 

 decomposable slates have mostly disappeared, leaving the underlying rock instead. If 

 we suppose, as seems most probable, that the limestone beneath has been elevated above 

 its normal position, it is probable that the reason why the slates do not appear over them 

 upon the east side, where nothing but the dove-colored limestone is found, is that the slate 

 is too deeply seated in the earth to be seen. Its whole thickness may be present, yet be 

 unseen. It is a region of disturbance ; and the slates may easily have sunk so low that 

 only the superior limestones appear. 



The thickness of the Hudson River Group upon Prof. Adams' section at Snake Moun- 

 tain is 930 feet. Deduct 70 feet for the limestone, and there remain 860 feet thickness of 

 slates and shales. The limestone we should estimate at more than a hundred feet in thick- 

 ness in the north part of Franklin County. We have paid scarcely any attention during the 

 progress of the survey to the measuring of the thickness of the rocks ; more particularly 

 because most of the formations in the State are so thick that neither geologists nor other 

 people would believe us if we stated the results in the Report. 



We have observed no minerals except calcite, and that in great abundance in veins, in 

 this formation. 



Geological Position and Equivalency. 



The rank of this group as the highest member of the lower silurian is well established 

 by Palaeontologists. The Snake Mountain and other sections, already presented, prove 

 the same thing. 



There is an item of interest under this head respecting the relative position of some of 

 the different members of the group itself or between the lower slates, and the upper 

 belt of pure white and dove-colored or less pure varieties of limestone and dolomite. The 

 most satisfactory sections are those that place these limestones at the top of the slates, 

 between them and the sandstones above. But some sections do not correspond to this 

 idea ; especially those at Rich's quarry in Swanton and in Highgate. There seems to be a 

 small thickness of slates intervening. This fact, however, may be regarded as settled that 

 these limestones occur only very near, or at the top of the group. It 'is a curious fact also 

 that often they form a wedge-shaped mass between the slate and sandstone. Another 

 fact is, that when the sandstone is replaced by dolomite, the two formations pass into 

 each other insensibly. 



ORGANIC REMAINS OF THE HUDSON EIVER GROUP. 



We have made scarcely any effort to obtain fossils from the Hudson River Group, and consequently our 

 knowledge of the life of this period in Vermont is exceedingly meager. We have seen in this rock 

 nndescribed species of Fucoids, several Graptolites, a species of Stromatopora (?), Chcetetes, Lycoper- 

 don (Hall), an undescribed species of Euomplialus, and the Triarthrus Beckii (Green.) 



