Prof. H. A. Nicholson — On the ' Guelph' Limestones. 345 



difficulty to obtain specimens that can be carried with comfort or 

 convenience. Chemically the Cedarville Limestone is an almost 

 typical dolomite, containing about 43 per cent, of carbonate, of mag- 

 nesia. In spite of this fact, it is reputed the best limestone in the 

 whole State of Ohio for liine-burning, its lime not being " fiery," 

 and taking much longer to " set," than is the case with lime made 

 from ordinary limestone. The thickness of the Guelph Limestones 

 in different parts of Ohio is estimated as varying between 20 and 

 90 feet. (Geological Survey of Ohio, Report of Progress in 1870, 

 p. 278.) 



Another development of the Guelph Limestones is known at the 

 Leclaire Rapids on the Mississippi River, in the State of Iowa, where 

 they were first recognized by Prof. Hall, and paralleled with the 

 Canadian series. (Geology of Iowa, vol. i. p. 73.) The beds consist 

 here of grey or whitish-grey magnesian limestones, of a semi-crys- 

 talline texture, and usually vesicular from the solution of fossils. No 

 lines of true bedding are visible, and consequently the thickness of 

 the series in this locality is unknown. Fossils are abundant, but 

 wholly in the condition of casts. A yellowish-grey concretionary 

 and false-bedded limestone identical with the above has been recog- 

 nized at Port Byron, in the State of Illinois, where it has a thickness 

 of about 50 feet. Limestones of the same age are likewise found in 

 the vicinity of Chicago, also in Illinois, where it has the same general 

 characters, except that it is highly charged with petroleum. ( Geolo- 

 gical Survey of Illinois, vol. i. p. 130 et seq.) Finally, limestones in 

 all respects identical with the preceding, and holding many of the 

 same fossils, have been recognized and described by Prof. Hall as 

 occurring at Racine, in the State of Wisconsin. (Geology of Wis- 

 consin, 1861.) 



Organic Remains. 



The organic remains of the Guelph Limestones are very numerous, 

 comparatively speaking, and are many of them peculiar ; but they 

 are badly preserved, and only certain groups have been as yet 

 worked out with any approach to fullness. The most highly 

 characteristic fossils of the formation belong to the groups of the 

 Brachiopoda, Lamellibranchiata, and Gasteropoda ; but it will be as 

 well to review briefly the more important forms which are known to 

 occur in these deposits, without attempting to give anything like an 

 exhaustive list. 



The only recorded species of Protozoa from the Guelph Formation 

 are two species of Stromatopora. One of these is the rare and 

 interesting S. ostiolata, Nich. ; the other is probably S. concenlrica, 

 Goldfuss ; but its state of preservation is such as to forbid a positive 

 specific determination. It is, however, an exceedingly common and 

 characteristic fossil in the Guelph Limestones of Canada, whole beds 

 appearing to be composed of very little else. In Ohio, I have not 

 noticed its occurrence at all. The only other fossil which could be 

 referred to the Protozoa, is a minute spherical body, of a calcareous 

 nature, which occurs in myriads in some portions of the Guelph 



