Prof. H. A. Nicholson — On the ' Guelph ' Limestones. 343 



II. — On the Guelph Limestones of North America and their 

 Organic Eemains. 1 



By H. Alletne Nicholson, M.D., D.Sc, F.E.S E., 

 Professor of Natural History in the University of St. Andrews. 



AMONGST the parallelisms which may be drawn between the 

 Silurian series of Britain and that of North America, none so 

 far has been so certainly established as the equivalency of the 

 "Niagara Formation" to the Wenlock Group. In its most typical 

 development, as in the State of New York, the Niagara formation 

 consists of an inferior series of argillaceous sediments, the " Niagara 

 Shales," and of a superior series of calcareous accumulations, the 

 "Niagara Limestone." At the Falls of Niagara itself, and at the 

 Falls of the Genesee at Eochester, the shales and limestones are 

 about eighty feet in thickness each. In Pennsylvania, the Niagara 

 formation is wholly shaly, and has a thickness of over fifteen hun- 

 dred feet. In the States west of New York, again, the formation is 

 almost wholly calcareous, many of its members being true dolomites, 

 and its total thickness rarely reaches three hundred feet, and is 

 usually much less. In Western Canada, finally, the Niagara shales 

 can rarely be detected as a distinct group, and the formation consists 

 mainly of limestones, often magnesian, with subordinate courses of 

 shale, the whole usually varying from one hundred to two hundred 

 feet in thickness. 



Whilst the above are the general characters of the Niagara forma- 

 tion as developed in North America, I purpose in the present com- 

 munication to discuss in greater detail a group of beds, which forms 

 the uppermost member of the series in its most complete develop- 

 ment, and which exhibits various points of special interest. The 

 beds in question have the lithological character of magnesian lime- 

 stone or dolomite ; and, though not universally present, they are so 

 constant in their position, and so sharply marked out by their 

 organic remains, that they have been raised by the Canadian 

 geologists to the rank of a distinct group, under the name of the 

 " Guelph Formation," from the town of Guelph, where they are 

 found in full force. This name has not been universally adopted ; 

 but it is convenient to use it, provided it be recollected that the 

 deposits in question are in reality only a portion of the great Niagara 

 Formation, of which they form the uppermost member. There are, 

 however, strong grounds for believing, with Sir William Logan and 

 Prof. Hall, that the Guelph Limestones, though found at points very 

 widely remote from one another, do not form a continuous series, 

 but that they are more or less of the nature of separate lenticular 

 masses of unequal extent and thickness, which have been deposited 

 on an uneven ocean bed towards the close of the Niagara period. 



The typical development of the Guelph Limestones is found in 



Western Ontario, where they were first noticed and described 



(Murray, Geol. Survey of Canada, Beport of Progress. 1848 and 



184:9 ; Hall, Pal. N. T. vol. ii. p. 340 ; Logan, Geology of Canada, 



1 Read before the Royal Physical Society, Edinburgh, Feb. 17th, 1875. 



