620 FRANK D. ADAMS 



sometimes almost massive and granitic, while elsewhere it is well 

 foliated. It sometimes occurs in great bodies free from all admixture 

 of other rocks, while at other times it is found intimately associated 

 or interbanded with the limestones or other rocks making up the 

 series. » 



The highly contorted character of this series of rocks made it 

 very difficult to determine its thickness, but Logan regarded the 

 following ascending section as representing approximately the suc- 

 cession and thickness of the various elements constituting the Lauren- 

 tian in the original area examined by him. 



FEET 



1. Anorthosite, above the Morin band of limestone, the thickness is 

 wholly conjectural 10,000 



2. Orthoclase gneiss, passing gradually into anorthosite, probably 

 includes the quartzite of Quartz Mountain, the anorthosite above it 



and the gneiss of passage - 3,400 



3. Proctor's Lake limestone 20 



4. Orthoclase gneiss IjS^o 



5. Crystalline limestone of Grenville, in some parts interstratified with 



a band of gneiss about. 75o 



6. Orthoclase gneiss, with several bands of garnetiferous gneiss and 

 quartzite, and with much coarse-grained porphyroid gneiss 3j5oo 



7. Crystalline limestone of Great Beaver Lake and Green Lake, includ- 

 ing two bands of interstratified garnetiferous rock and hornblendic 

 orthoclase gneiss 2,500 



8. Orthoclase gneiss - 4,000 



9. Crystalline limestone of Trembling Lake i>5oo 



10. Orthoclase gneiss composing Trembling Mountain, lower limit not 



ascertained, thickness probably exceeds S>ooo 



32,250 



The base of the series was thus a great body of orthoclase gneiss 

 — the Trembhng Mountain gneiss— above which were four beks of 

 limestone, certain of them of great thickness, akernating with ortho- 

 clase gneiss; the whole succeeded by a great development of anor- 

 thosite. 



The ahernation of limestone with gneiss was considered by Logan 

 as proving that the whole series represented a great body of highly 

 altered sediments, the oldest sediments recognizable in the earth's 

 history. The foHation exhibited by the bodies of orthoclase gneiss 



