300 American Scientific Association. 



ON THE LAURENTIAN LIMESTONES. 



Sir William Logan exhibited to the section, a map on which 

 was delineated in detail on the scale of an inch to a mile, the dis- 

 tribution of some of the bands of crystaline limestone interstra- 

 • titled with the gneiss of the Laurentian series of rocks on the north 

 side of the Ottawa River, about forty miles above Montreal. This 

 he explained was a continuation of similar work shown at the 

 Montreal meeting of the association. By his recent exploration, two 

 additional 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 15000 feet thick. These three bands are 

 separated from one another by gneiss, a large portion of which is 

 porphyroid or coarse-grained, the feldspar being almost wholly 

 orthoclase, whereas, as was stated at the Montreal meeting, cal- 

 careous bands above them are largely associated with labradorite. 

 Intercalated with the coarse and massive orthoclase gneiss, were 

 frequent beds, which may be characterized as mica slate, and 

 approaching the calcareous bands are beds of hornblende rock, 

 and quartz rock, these latter, and sometimes bands of nearly pure 

 white orthoclase, when immediately near the limestone or inter- 

 stratified with it, being very often thickly studded with pink gar- 

 nets, one of the beds of white and nearly pure quartz rock, which 

 was traced for a mile and a-half, presented a thickness of 1000 

 feet. No instance of clay slates was met with. 



These ttrata are exceedingly corrugated, and the outcrop of the 

 limestone presents a multitude of sharp turns resulting from small 

 plications subordinate to more important synclinal and anticlinal 

 forms, the axes of which appear to run nearly north and south. 

 Some of these axes have now been traced up the Rouge, a tributary 

 of the Ottaw r a, for a distance of fifty miles in a straight line. 



Althouo-h the Laurentian series has hitherto been considered 

 azoic, a search for fossils in them has not been neglected. Such 

 search is naturally connected with great difficulties. Any organic 

 remains which may have been entombed in these limestones, would, 

 if they retained their calcareous character, be almost certainly ob- 

 literated by crystalization, and it would only be through their re- 

 placement 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 investigations of the Rouge and its 



