268 



GEOLOGY OF THE SECOND DISTRICT. 



Liiigula anliqua. 



It is at this place that the Lingula antiqua is found in sucli great 

 abundance, being common in the strata to the depth of about seventy 

 feet. It is, however, extremely obscure. Wherever it exists between 

 the layers, it may be known by the dark colored lines which appear 

 on a transverse fracture. It is extremely thin and delicate, and by 

 far the greatest number appear only in a very imperfect state, and, 

 \ i V ■■ i in consequence, have been frequently overlooked, or taken for a thm 



\J^^^^^^^^ film of carbonaceous matter. The medium in which they lived was 

 one which furnished only a small amount of carbonate of lime, a 

 material very essential for the habitations of all testaceous animals. 

 It is the lowest and oldest fossil now known, and the genus to which it belongs has survived 

 all the changes upon the earth from the era of the formation of this rock to the present time. 

 The surface of most of tlie layers of the potsdam sandstone appears with ripple marks, 

 particularly at Port Kent, where they are remarkably fine and deep. At the village of Keese- 

 ville, the rock is used extensively as a building material ; it is, however, the crystallized 

 variety, and hence does not break so readily into those rectangular forms, as the same mass 

 at the Potsdam quarries in St. Lawrence county. 



From Keeseville, it extends north several miles, but is very much concealed by beds of 

 sand and gravel, such as may be seen immediately north of the mouth of the Ausable. This 

 place is the most northern point in the State, upon the borders of the lake, at which the 

 potsdam sandstone occurs. Towards Plattsburgh the limestone and shales of the Champlain 

 group succeed, with a dip to the north ; and so far as I have observed, or can learn from 

 others, it does not appear between Port Kent and Quebec. Upon the northern slope, near 

 the provincial line in Champlain, and also west of Plattsburgh towards Redford, it forms 

 extensive and important masses, of which I shall have occasion to speak under the head 

 of Clinton county. 



From the preceding observations, it will be seen that Keeseville is the most important 

 locality of this rock in Essex county, both as it regards thickness and extent of surface ; for 

 I shall soon have occasion to speak of the continuous range of this mass, after passing into 

 Clinton county. In fact, we shall find that it is one connected rock, sweeping around and 

 occupying the northern slope of New-York, along the provincial line, into Franklin and St. 

 Lawrence counties. 



I could not ascertain the thickness of this rock at any place which fell under my observa- 

 tion : at the High bridge, it is known to be over one hundred feet from the water below, or 

 from the river to the top of the rocks above, but this is considerably less than the whole thick- 

 ness. I have often had occasion to notice gorges of this description, at many other places in 

 this district. From present appearances, I am disposed to attribute their formation to a 

 slight fracture, and upheaval of the mass in a given line or direction. Into a fracture thus 

 formed, a river, from some cause or other, happens to fall : the fracture then becomes the natu- 

 ral channel, and, in process of lime, the river undermines, breaks up, and sweeps out all loose 



