INVESTIGATIONS OF SIR W. LOGAN. 353 



certainly known whether the calcareous strata are confined to one group, often 

 repeated by sharp undulations, or whether it is probable there are several groups, 

 separated by heavy masses of gneiss. . . . The dip avails but little in ascer- 

 taining this, for in the numerous folds with which the formation is wrinkled these 

 dips must frequently be overturned, and the only reliable mode of pursuing the 

 investigation and of making even the limestone available for working out the 

 structure is to patiently and continuously follow out the outcrop of each important 

 mass in all its windings, as far as it can he traced, until it becomes covered up by 

 superior unconformable formations, is cut off by some great dislocation, or disap- 

 pears by thinning away to nothing." 



The attempt Avas therefore made in this district to work out the struc- 

 ture of the Laurentian limestone bands by tracing out their outcrops, an 

 undertaking which, in the unsettled and wooded character of most of this 

 region, was to a large extent impossible of accomplishment, especially in 

 a country to a large degree occupied by rugged ranges of mountains. To 

 make the work still more difficult and unsatisfactory, Sir William was 

 obliged to entrust it to men entirely without scientific training of any 

 kind and ignorant of the simplest points in regard to geologic structure, 

 more particularly when overturned strata or distortions, arising from the 

 presence of intrusive masses, complicated the problem. But little at- 

 tempt was made by his assistants to record strikes and dips, and, as the 

 outcrops were frequently concealed by drift over long areas or terminated 

 by thinning out or faulting, the joining of widely separated points, on 

 the hypothesis that these represented portions of the same continuous 

 band, of necessity produced a structure which was difficult to clearly 

 comprehend. 



Summary of Sir W. Locian's Laurentian Suction. 



The structure of the Laurentian deduced by Logan, largely from the 

 labors of his assistants, and summed up in the statement given in Geology 

 of Canada, 1863, known as the Trembling Mountain and Lake section, 

 may be summarized as follows : 



1. The mass of orthoclase gneiss composing Trembling mountain, 



thickness unknown, estimated 5,000 feet. 



2. Crystalline limestone of Trembling lake 1,500 " 



3-9. Four other bands of orthoclase gneiss in masses of 1,580 to 4,000 



feet, separated by hands of crystalline limestone, in thickness 



respectively from 20 feet to 2,500 feet, in all 15,750 " 



10. Anorthosite, into which the upper band of orthoclase gneiss was 



supposed to pass, thickness entirely conjectural 10,000 " 



The whole supposed to present an ascending series ami aggregat- 

 ing 32,250 " 



