176 GEOLOGY [CHAP. IX 



Letter 518 but owing to the want of a true ground-plan it \vas impossible 

 to ascertain their exact curvature, which, as far as could be 

 estimated, appeared equal to that of the surface of the sea. 

 Considering how very rarely the sea has left narrow and well- 

 defined marks of its action at any considerable height on the 

 land, and more especially considering the remarkable observa- 

 tions by M. Bravais l on the ancient sea-beaches of Scan- 

 dinavia, showing that they are not strictly parallel to each 

 other, and that the movement has been greater nearer the 

 mountains than on the coast, it appears highly desirable that 

 the roads of Glen Roy should be examined with the utmost 

 care during the execution of the Ordnance Survey of Scotland. 

 The best instruments and the most accurate measurements 

 being necessary for this end almost precludes the hope of its 

 being ever undertaken by private individuals ; but by the means 

 at the disposal of the Ordnance, measurements would be easily 

 made even more accurate than those of M. Bravais. It would 

 be desirable to take two lines of the greatest possible length 

 in the district, and at nearly right angles to each other, and 

 to level from the beach at one extremity to that at the other, 

 so that it might be ascertained whether the curvature does 

 exactly correspond with that of the globe, or, if not, what is 

 the direction of the line of greatest elevation. Much attention 

 would be requisite in fixing on either the upper or lower edge 

 of the ancient beaches as the standard of measurement, and 

 in rendering this line conspicuous. The heights of the three 

 roads, one above the other and above the level of the sea, 

 ought to be accurately ascertained. Mr. Darwin observed 

 one short beach-line north of Glen Roy, and he has indicated, 

 on the authority of Sir David Brewster, others in the valley of 

 the Spey. If these could be accurately connected, by careful 

 measurements of their absolute heights or by levelling, 

 with those of Glen Roy, it would make a most valuable 

 addition to our knowledge on this subject. Although the 

 observations here specified would probably be laborious, 

 yet, considering how rarely such evidence is afforded in any 

 quarter of the world, it cannot be doubted that one of the 

 most important problems in Geology namely, the exact 



1 " On the Lines of Ancient Level of the Sea in Finmark," by M. A. 

 Bravais, translated from Voyages de la Commission Scientifique du Nord, 

 etc.; Quart. Journ. GeoL Soc. t Vol. I., p. 534, 1845. 



