622 FRANK D. ADAMS 



and in part represent highly altered sediments. It is impossible in all 

 cases to distinguish these two classes of rocks, but the distinction can 

 in many cases be made with certainty. 



The sedimentary gneisses (paragneiss) are fine in grain and 

 usually occur intimately associated or interstratified with the lime- 

 stones and quartzites, and very frequently weather to a rusty color. 

 From the thickness of the limestone bands given above, there must 

 be deducted ihat of the igneous gneiss included in or associated with 

 them. 



It is also by no means certain that the four limestones mapped by 

 Logan are all separate bands, seeing that the intervening belts of 

 gneiss, being largely igneous in origin, can no longer be regarded as 

 representing separate stratigraphical elements, but may be intrusions 

 separating portions of one and the same body of limestone. Whether 

 or not this latter presumption represents the truth, can, however, 

 only be determined by a complete re-examination of Logan's work 

 in this area. In the meantime it is certain that the highest and 

 lowest members of his series are bodies of intrusive rock in which a 

 foliation has been induced by pressure. The deduction of these 

 reduces the thickness of the series by 15,000 feet, and that a much 

 greater reduction than this must be made if the true thickness of the 

 sedimentary portion of the series is to be ascertained, is evident from 

 what has been already stated. 



In his map of this area, Logan separated the anorthosite and 

 designated it as Upper Laurentian, while the other rocks of the 

 series were classed as Lower Laurentian or Grenville series. In 

 after years the lowest gneiss came to be known as the Fundamental 

 Gneiss or Ottawa Gneiss, while the name "' Grenville series " was 

 restricted to the limestone-bearing portion of Logan's Laurentiau. 



This work of Logan, while in many respects imperfect, was 

 nevertheless a first approximation to a true knowledge of the Lauren- 

 tian and served as an excellent basis for the work of subsequent 

 investigators. 



Logan subsequently found in eastern Ontario a series of rocks 

 which he considered in all probability to represent the Grenville 

 series in a less altered form, and to these he gave the name of the 

 "Hastings series." 



