190 Correspondence — B. Smith Lyman. 



is at their base. This will prove, if the line of argument at present 

 in use be allowed, tbat all their recent accumulations, and even the 

 railways, are pre-glacial. I have seen from ten to twenty feet of as 

 good Glacial Drift as that from which the existence of the Middle 

 Gravels have been proved (?), covering a recent railway, or some 

 other modern structure ; and I have heard such covering pronounced 

 " good typical Glacial Drift " by an eminent geologist before he 

 was pointed out what was beneath it. 



G. Henky Kinahan, 

 "Wexford, Feb. 6, 1875. Irish Branch, H.M. Geol. Survey. 



GEOLOGICAL SUKVEY OF TESSO. 



. Sir, — While thanking you for the kindly notice (in the last re- 

 ceived number of your Magazine, October, 1874) of my little report 

 of a year ago on the first season's field-work of the Geological Survey 

 of Yesso, I beg to make a correction in the criticism on the topo- 

 graphical-geological method of Prof. Lesley (chief of the new Penn- 

 sylvania Geological Survey). He should not be blamed for the 

 " confusion and unsightliness " of the lines on a map that shows the 

 contours of the principal beds of rock as well as of the surface ; for 

 his maps are models of clearness and taste, and even on a large scale 

 commonly show for the rocks only the outcrop and the lowest natural 

 drainage level of the beds of chief mining importance, and the topo- 

 graphy is often reinforced by shading, besides the contour-lines. The 

 addition of contour-lines for such beds above water-level, and to a cer- 

 tain depth below, is my own idea, and what I fondly imagined to be an 

 improvement, especially in mapping limited tracts of land where the 

 owners wish to see at a glance as by a sort of cross-hatching on the 

 map what portion of the ground is underlain by workable beds. In 

 many regions, perhaps most, it is possible to draw such underground 

 contour-lines with a degree of accuracy very useful for practical 

 mining purposes (one coal-bed, for example, was shown by a map to 

 be at 180 feet below the surface of the ground at a point three- 

 quarters of a mile from the nearest exposures of the bed, and on 

 sinking a pit proved to be at 182 feet). The rocks are not in every 

 country tied up in double bow-knots, as they sometimes seem to be 

 in the Himalayas. Of course it is difficult to trace out such contor- 

 tions, or to represent them on a map in any way ; for even every small 

 irregularity in the surface-contours cannot be given on maps of small 

 scale. 



It must be admitted that to draw two sets of contour-lines on the 

 same map, especially if both are black for photographing, necessarily 

 takes away somewhat from the good appearance of either alone ; but 

 is there not some compensation in the additional information con- 

 veyed, and in the display of the relation of the surface-contours to 

 the underground contours at every point ? It must also be acknow- 

 ledged that " observations made at the surface can only be taken for 

 what they are worth," and the underground contours of a bed of 

 rock must always be somewhat less certain than those of the surface. 

 Still, is it not worth while for the observer to give precisely what, 



