356 K. W. ELLS— LAURENTIAN OF THE OTTOWA DISTRICT. 



those cases where any considerable quantity of limestone appears to be 

 overlaid by gneiss in regular sequence, such superposition of the gneiss 

 is due to overturned strata, and thai sometimes gneiss is brought against 

 the calcareous measures by lines of fault. Abrupt changes of dip and 

 strike arc frequent, and in occasional sections, displayed along the shores 

 of some of the larger inland lakes, the repetition of the folds into well- 

 defined synclinals of limestone, separated by anticlinals of gneiss, occur 

 formany hundreds of yards and reveal this feature of the structure very 

 clearly. 



Further, it has been found impossible to trace any particular band of 

 limestone to any considerable distance continuously. Masses of lime- 

 stone are often local in their development, presenting frequently lenticu- 

 lar forms which are thick near the center and thin off toward the extrem- 

 ities. They are often terminated abruptly by dislocations of the strata 

 or by the intrusion of other rock masses, and frequently the synclinal 

 structure in the gneiss can be seen for a long distance after the ending 

 of the calcareous overlying portions which may have been removed by 

 by denudation. In certain areas the synclinals follow one another in 

 quick succession, while the limestone maybe exposed for only a few 

 hundred yards or feet in each. In many places also there is a very 

 heavy covering of clay drift and sand, which conceals both the gneiss 

 and limestone. The latter is rarely seen except in valleys, the hills 

 where not of intrusive syenite or anorthosite being of the harder and 

 more feldspathic variety of gneiss, often interlaced with intrusions of 

 feldspathic rock or pyroxenic dikes. 



While it is impossible to give in a paper of this kind such data as 

 strikes and dips on which the theory of structure here presented rests, it 

 may be here stated that throughout the section of eighty miles or more 

 from east to west the limestone occupies the synclinals in the gneiss 

 almost without exception. It will, however, be understood that in the 

 lower part of the calcareous portion certain thin bands of limestone are 

 interstratitied with the grayish and rusty gneiss which forms the upper 

 portion of the stratified gneiss series, and thus a gradual upward passage 

 from the gneiss into the limestone is presented, but in no observed case 

 are these interstratifications of gneiss of any great thickness, and their 

 relations to the overlying calcareou- beds can be easily recognized. 



The Laurentian Gneiss and Limestone. 



Their Thickness. — It is, of course, almost impossible to arrive at any 

 correct conclusions as to the thickness of the gneiss or limestone in a 

 series so twisted and so faulted as the Laurentian. Perhaps the best 

 section for this purpose is found on the Rouge river from a point about 



