326 Dr. Mac Culloch on the Parallel Roads of Glen Roy. 



parallelism in the horizontal one, and causes, as will be readily 

 comprehended, the distance between any two approximate ones to 

 vary exceedingly, a circumstance of importance in examining some 

 of the speculations which have been formed relating to their cause 

 and origin. In these circumstances it would be as useless as it would 

 be difficult, to give the measures of their variations. It is sufficient 

 to remark that the nearest horizontal distance between the upper- 

 most and nearest lines may be taken at 150 or 160 feet, and the 

 greatest between the two lowermost and most distant ones at 1000; 

 an approximation sufficiently accurate for the purpose. But the 

 vertical distances as ascertained by the spirit level are 82 feet be- 

 tween the uppermost and second, and 212 between the second and 

 lowermost. I have omitted fractions in this measurement, because 

 the irregularity of the ground is such, and the uncertainty of the 

 true surface of the line so great, that it is impossible to determine 

 this point to a great nicety. The slope of each line is likewise so 

 considerable, and at the same time so various, that a great variation 

 of this vertical distance would take place according to the point 

 adopted as the station of the spirit level, and I have therefore fixed 

 it at the middle of each line. I need not here call the reader's at- 

 tention to the distances between the uppermost line and the top 

 and bottom of the glen respectively, as these will fall more properly 

 to be considered when the general levels of the surrounding vallies 

 and outlets are examined.* For the same reasons the aspect and 

 materials of the lines themselves will be best considered when I 

 have described the mineralogical structure of the whole glen, and 

 particularly that of the surfaces on which they are formed. 



* As a convenient reference to the reader, I have however thought proper to tabulate 

 in one view all the measurements relating to this subject which are in any way interesting; 



