POSITION AND EXTENT OF COAL BEDS. 81 



of from a few score to 600 or 800 feet it may pass from a mere trace to the 

 thickness of 20 feet or more, the cross section having a rudely lenticular 

 form. Followed horizontally, these thick portions of the vein thin toward 

 either end — at least that is the impression made by a study of the Ports- 

 mouth mine. So far as could be seen there, the horizontal dimension of the 

 lens was much greater than that shown in descending the slope. It is 

 evident that these conditions exclude any careful study as to the thickness 

 of the beds. It may be said that a rough computation of the contents of 

 the principal bed mined at Portsmouth showed it to be probable that the 

 thickness of the deposit before it was disturbed by the shearing action was 

 not far from 4£ feet. 



It need not be said that this irregular form of the coal deposits, com- 

 bined, as it is, with a certain amount of faulting, which, though not dis- 

 tinctly shown in the small workings, is evident in the structure of the field, 

 makes it important to determine how far these features are general through- 

 out the basin. On this point the information is very scanty. It may be 

 said, however, that where, as in the Portsmouth mine, the workings had gone 

 for a distance of about 1,400 feet from the outcrop, and where the steepness 

 and the dip considerably lessened with the approach to the center of the 

 syncline, the irregularity of the bed had perceptibly diminished, giving 

 some reason to expect that there was an extensive area of coal in that cen- 

 tral part of the trough which had not been much dislocated. Unfortunately, 

 this is the only portion of the basin where there is sufficient basis for reck- 

 oning that the coals within reach of mining work occupy a position which 

 gives them the chance of escaping the effects of "rolling." 



Owing to the lack of detailed knowledge concerning- the position of 

 the coals, or even of the precise attitude of the rocks in this basin, it is not 

 yet possible to estimate with any approach to accuracy the area in which 

 coals of workable thickness may be found. It may be said in general that 

 all parts of the section lying more than 2,000 feet below the base of the 

 upper conglomerates show, from point to point, traces of coal. Consid- 

 ering the numbers of these chance exposures, and noting the general way 

 in which the portions of the section containing coal are hidden by glacial 

 detritus, there is reason to believe that a considerable part of the rocks 

 below the indicated level are in some measure coal bearing. Definite 

 mon xxxiii 6 



