Petrographical Relations of Laurentian Limestones. 27 



What is the source of the various basic silicates which 

 make up the dark bands in the limestone ? The principal 

 materials which must have been added to the elements of 

 limestone to produce the various silicate minerals observed 

 in the altered limestones are : Silica, alumina, iron, 

 magnesia, alkalies, and chlorine (for scapolite). As to 

 the source of these materials, there are two possible 

 explanations ; either they existed in the limestone or in 

 beds interstratified with it before the intrusions took 

 place, and were subsequently metamorphosed and re- 

 crystallized ; or they have been derived in some manner 

 from the substance of the intrusive mass. The question 

 resolves itself, then, into one of metamorphism by 

 diagenesis, or by metasomatosis. 



It has been previously stated that the limestones are 

 in general comparatively pure except as these igneous 

 masses are approached. It may be noted in the first place 

 that the peculiar manner of occurrence of these basic bands 

 in the limestone is hardly in accord with the ordinary 

 observed phenomena of sedimentation. Secondly, there 

 seems to be no reason, supposing that the above mentioned 

 materials, which may for the present be called impurities, 

 were already contained in the limestone, why the igneous 

 material should invariably choose just that locality for 

 intrusion. And it is further impossible to see in the case 

 of the pyroxene gneisses, how that supposition would 

 account for the very general increase in the degree of 

 metamorphism as the intrusion is approached, from pure 

 limestones to such basic rocks. Moreover, if these im- 

 purities existed in the beds of the limestone in this region, 

 it would be quite natural to suppose that other impurities, 

 giving rise to different metamorphic products, might occur 

 in other limestone regions similarly pierced by igneous 

 intrusions. It will be shown, however, that such is 

 generally not the case. 



Too much importance can hardly be attached to the 



