Apatite Bearing Rocks. 215 



The apparent conflict of opinions on this subject is 

 doubtless to some extent due to the diverse views which 

 have been held regarding the composition and structure of 

 the Laurentian rocks themselves. These, in the early days 

 of their study, were supposed by Sir Wm. Logan and his 

 co workers to be very largely sedimentary in their nature, 

 and formed just as are the sandstones and fossiliferous 

 limestones of more recent formations. In the stratified 

 sedimentary complex, was included not only the crystalline 

 limestone, gneiss and quartzite, but also the anorthosites, the 

 pyroxenes, feldspars and many of the syenitic rocks which 

 subsequent careful study in the field and laboratory have 

 shown to have originated in a very different manner. That 

 the greater part of the latter group is intrusive in the strati- 

 fied gneiss and limestones is now very clearly established, 

 while the present highly altered condition of the gneiss 

 and associated limestone is doubtless due, in part at least, 

 to the great processes of metamorphism which have taken 

 place during the ages subsequent to their deposition as 

 also, to some extent, to the action of the subsequent in- 

 trusive masses. 



It is impossible here to go into any elaborate discussion 

 as to the origin of the Laurentian rocks, further than may 

 be absolutely necessary for a clear understanding of the 

 subject under discussion. It may however be said from the 

 evidence at present at our disposal that they have been pro- 

 duced in two ways ; for while it is clear that a very large 

 proportion, by far the largest in certain areas, possesses the 

 characters of igneous rocks, certain well stratified areas 

 of limestones with associated quartzite and gneiss, present 

 very many* of the features of sedimentary rocks, especially 

 in their present arrangements, and have presumably been 

 deposited through aqueous agencies. The evidence from 

 organic remains usually found in sedimentary rocks, is, 

 however, wanting, but this may be due to the absence of 

 life on the globe in those early periods of the world's his- 

 tory, for, if such existed, their traces have entirely disap- 

 peared from some cause not yet definitely known. 



