90 Canadian Record of Science. 



enclosing limestone ; at places this tapering off is seen, on 

 closer inspection, to be due to strings of particles of fel- 

 spathic and similar material arranged in line. 



All these inclusions of whatever shape seem to have one 

 feature in common. Their exterior surface it hackly, pit- 

 ted and with a generally corroded appearance. 



The general striping of the limestones would seem, on 

 careful study, to be due, either to little irregular chains of 

 such particles or to a different colouring of the replacing 

 calcite crystals, probably marking the places where such 

 particles have been. 



The detailed explanation offered by the writer seems to 

 him to satisfactorily and thoroughly explain these and 

 other features of the limestones of the Laurentian. He is 

 led to the conclusion that they simply represent areas of 

 gneissic and similar rocks altered in place into limestones. 

 Furthermoie, that the extent and location of these areas 

 have been largely determined by the presence of the abrupt 

 bends and other contortions of these rocks, whose foliar 

 would thus be separated and opened up to the complete 

 action of the subterranean waters. Where such contor- 

 tions and crumpling of the rocks had extended over a con- 

 siderable area, the alteration would have gone further and 

 have produced the solid limestone masses so frequently 

 found. In these the inclusions would naturally be scarcer 

 and represent the more solid portions of the ribs of the 

 gneiss which for this reason or owing to their mineral 

 composition were less amenable to change than the rest of 

 the area. These would naturally show the corroded sur- 

 face already alluded to, and the tapering off along the strip- 

 ing of the rock. The lesser and scattering occurrences of 

 limestone throughout the district, which are a very confus- 

 ing feature on any other supposition, would thus be satis- 

 factorily accounted for as well as the extreme irregularity 

 of the boundaries of these limestone areas and other 

 phenomena of their occurrence. 



Doubtless also the mineral constitution of the original 

 rocks must have been an important factor in the determina- 

 tion of the position, etc., of these alteration areas. 



