THICKNESS OF THE LAURENT TAN LIMESTONES. 273 



importance to be represented separately on the map."* 

 At the meeting of the same body in 1859, Sir William 

 Logan exhibited a map on which was delineated in detail 

 the distribution of some of the bands of crystalline 

 limestone interstratified with the gneiss of the Laurentian 

 rocks on the north side of the Ottawa river, about forty 

 miles from Montreal, being a continuation of the work 

 •referred to at the Montreal Meeting in 1857. Two ad- 

 ditional bands of limestone had been ascertained to 

 underlie the lowest of those previously examined, the 

 whole of the strata associated with these lower three, 

 including the limestones, being supposed to be about 

 15,000 feet thick. f 



* Canadian Journal; January, 1858. 



f Until lately the Potsdam sandstone lias been supposed to represent the 

 epoch when organic life was first introduced by the Creator on the surface of 

 our globe. Recent discoveries tend to throw back the first peopling of the 

 world into a past so indefinitely remote, that all preconceived ideas of 

 organic history become unsettled and at fault. The following notice is from 

 Tlie Canadian Naturalist and Geologist, vol. iv. p. 300 : — 



" Although the Laurentian Series has hitherto been considered azoic, a 

 search for fossils in them has not been neglected. Such search is naturally 

 conducted with great difficulties. Any organic remains which may have 

 been entombed in these limestones, would, if they retained their calcareous 

 character be almost certainly obliterated by crystallization, and it would be 

 only through their replacement by a different mineral substance that there 

 would be a chance of some of the forms being preserved. No such instances 

 had been observed on the investigation of the Rouge and its vicinity, but 

 from another locality in the Laurentian formation, Mr. John McMullen, one 

 of the explorers of the Geological Survey, had obtained specimens well 

 worthy of attention. They consisted of parallel or apparently concentric 

 layers resembling those of the coral Stromatocerium, except that they anas- 

 tomose at various parts ; these layers consist of crystalline pyroxene, while 

 the interstices are filled with crystallized carbonate of lime. These specimens 

 had recalled to recollection others which had been obtained from Dr. Wilson 

 of Perth some years ago, and had not then been regarded with sufficient 

 attention. In these similar forms are composed of green serpentine, concre- 

 tionary, while the interstices are filled with white dolomite. If it be sup- 

 posed that both are the result of mere unaided mineral arrangement, it 

 would seem strange that identical forms should result from such different 



VOL. II. T 



