LOWER HELDERBERG LIMESTONE. 391 



From this it appears that it differs from the rock iipon Mud creek 

 very slightly, except in its content of oxide of iron, which gives to it 

 its dark brown color. There can be no doubt then that this is to be 

 regarded as constituting the uppermost portion of the formation un- 

 der consideration, which, therefore, immediately underlies the 1-j.am- 

 ilton strata. The exposure upon the Milwaukee river is very slight, 

 and it is impossible to determine whether there is strict conformabil- 

 ity between the Hamilton rock and that under consideration or not. 

 It can only be said that if any unconformability exists, its amonnt is 

 very slight, as the dip of the strata do not markedly differ. 



Extent. These are all the exposures of this rock at present known 

 immediately adjacent to the Hamilton formation. Unfortunately al- 

 most the whole of this region is thickly covered with drift, and ex- 

 posures of rock are exceedingly rare. All the territory that can safe- 

 ly be regarded as occupied by this shaly limestone will be found in- 

 dicated by gray checks on the accompanying map. As hereafter 

 stated, it is probable .from drift evidences that there is a small patch 

 on the north side of the Hamilton area, but its precise location is un- 

 known. It might be presumed that the formation would occupy a 

 belt surrounding the cement rock, a conjecture which has heretofore 

 found expression. But the following facts forbid such an hypothesis. 

 At Schwartzburg, a mile north of the outcrops on Mud ci-eek, rock 

 was reached in the excavation of a cellar, which presents all the litho- 

 logical characteristics of the Niagara limestone, and contains Pen- 

 tamerus ventricosus, a ITiagara species. 



In the ]Sr. W. qr. of Sec. 10 and the IS". E. qr. of Sec. 9, of the town 

 of Granville, we find the most northwesterly known exposure of the 

 overlying Hamilton formation, in the brow of a hill facing to the 

 northwest. Only thirty-six paces down the gentle slope, from the Ham- 

 ilton beds, a pit has been opened which discloses the Niagara lime- 

 stone. The vertical distance between the top of the Niagara exposed, 

 and the bottom of the quarry of Hamilton rock is about six feet. 

 The intermediate slope is largely occupied with old pits, now filled, 

 but in the material thrown from them, only Niagara and Hamilton 

 rock was seen. In the gutter of the adjacent road, both the Hamil- 

 ton and Niagara are shown, with a vertical distance of less than five 

 feet between them, and in the abundant chipstone of the gutter, there 

 was none of the shaly limestone under discussion. But it is a rock 

 peculiarly liable to break up into chipstone, and is abundant in the 

 drift near the known outcrops, and in the line of drift from them. 



In view of all these facts, the shaly limestone must be regarded as 

 absent at this point. Tlie occurrence of the Niagara limestone along 



