Perry.] 348 [December 18, 



dence in the organic remains, tliat these groups of strata belong to the 

 same specific life-period. 



It slionld accordingly seem, from this three-fold lack of conformity, 

 that the beds underlying the Red Sandstone are distinct from it. 

 Indeed, the facts brought forward make it appear that the lower 

 rocks cannot be, with any propriety, either referred to the formation, 

 or regarded as belonging to the period called Potsdam. 



IV. What relation, then, we may now ask, does the Ked Sand- 

 stone-sustain to the underlying fonnations ? 



The true answer to this query must depend upon the determination 

 of the age of the inferior rocks in question. These, and especially 

 portions of the formation sometimes called Black Slate, were long and 

 generally, though by no means universally, i-egarded as Utica Slate and 

 Lorraine Shale. They should be divided, as is most probable, into 

 two ]nrts, and may be locally designated as the Swanton Series and 

 the Georgia Series. The western, or Swanton, portion is usually very 

 dark, or black, and hence the name Black Slate, by which it is often 

 known; that next to it on the east, or the Georgia range, is for the 

 most part brown; while both groups contain interstratified layers of 

 sandstone and limestone. The fact that beds of brown sandstone 

 occur in connection with the slate, sometimes clearly interstratified 

 ■with it, has led many inadvertently to refer them — indeed, often to 

 consider the whole of these inferior formations as belonging — to the 

 Pots.Lim series of rocks, even in localities in Avhich the lower and 

 the upju'r groups may be seen to be unconformable, and therefore, as 

 has been already shown, clearly distinct from each other. 



That these lower formations are not of so late an age as the Utica 

 Slate and the Lorraine Shale is apparent, both from what has been 

 said of the Red Sandstone which ovei'lies them, and from other con- 

 siderations. In some cases, the Black or Swanton Slate may be seen 

 bene;th the Potsdam Sandstone, not only along its western flank, but 

 also at the very edge of its eastern hmits. After long searching, I was 

 at last so fortunate as to find the two rocks thus situated, and in im- 

 mediate conjunction. This was on the easterly border of the sand- 

 stone, at Shelburn Falls, Avhere, some years ago (in the summer of 

 1860), an excavation was made in the channel of the La Platte 

 River. Since that time, I have observed substantially the same 

 thing at many other points. So the Georgia. Slate may be traced 

 beneath the Potsdam Sandstone Avith equal clearness, and shown to 

 underlie it, in its extreme extension eastward in Swanton; while the 

 two rocks may be seen to sustain virtually the same relations in St. 

 Albaus, in liighgate, and in fact in not a few other localities in west- 

 ern Vermont. These facts, with their conconutants, it is thought, 

 entirely meet and set aside the supjiosition still sometimes made, that 



