AS one of your reviewers now some years ago criticized unfavourably the method of representing geological structure in maps by contour lines upon a bed of coal or other rock, and as, like him, most geologists are not yet practically familiar with the method, useful though it be, and nobody that might be thought wholly unbiassed has replied in your pages to his attacks, it may not be improper to call your attention to the unprejudiced opinion and explanation of Mr. Charles A. Ashburner, the geologist in charge of the survey of the anthracite region, under the Geological Survey of Pennsylvania, who has up to this time published 26 large sheets of most elaborate maps and sections with one volume of text. After careful examination before adopting the method and after using it several years he writes of it in the report (page 8) as follows:—