EDITORIAL WORK ()N GEOLOGIC MAPS. 103 
to run through the printing presses a .sheet with '2~i colors, averaging 
about one color a day — the changes in humidity may be very great and 
are known to have caused a change of one-eighth of an inch in the 
length of a sheet. To overcome this trouble as far as possible the 
room in which the partly printed sheets are stored is kept at uniform 
temperature, the windows being closed the year round. A humidity- 
recording instrument is also kept in the room as an indicator of 
conditions. 
The structure and columnar sections are usually prepared by the 
draftsmen of this section of the Survey, who copy the outlines of the 
draft submitted by the authors, but use a definite system of litho- 
logic symbols to represent the kinds of rock, in this way obtaining 
uniformity of style, and symbols. Thus, dots represent sand, sand- 
stone, and quartzite, the coarseness being indicated by the size of the 
dots; parallel dashes represent clay, shale, and slate; parallel lines 
united here and there by short perpendicular lines to form rectangles 
represent limestone; irregularly placed dashes represent igneous 
rocks. For rocks intermediate between these types the appropriate 
patterns are combined.. 
The structure sections are engraved and transferred to blank strips 
left for them on the structure-section sheet, the outcrop of the 
formations on the sections corresponding with the boundaries on the 
map along the lines of the sections. Small strips of the lithologic 
Symbols used in the sections are engraved in the corresponding blocks 
of the legend. Sheets are then colored for the sections similar to those 
prepared for the maps, and the color proofs are read. A portion of 
one of the geologic maps with section is shown in PI. II (p. 30). 
The cover of the folio not only serves the purpose of protecting the 
contained sheets, but bears on its front page an index map showing the 
location of the area mapped, and on its inside pages a description of 
the topographic map and a brief outline of geologic processes, intended 
to prepare the lay reader to understand the more detailed and some- 
what technical description within. 
The first of the final series of folios was completed in 1894. In the 
succeeding ten years 106 folios have been published, an average of 
over ten a year. Three of the folios each include maps representing 
four contiguous areas, and eight folios embrace two such contiguous 
areas each, so that in labor and time they represent a correspond- 
ingly larger number of ordinary folios. Seven folios contain special 
detailed maps of mining regions, and two more are in process of 
publication; eleven have special artesian-water maps. Three folios 
embracing large cities (New York. Chicago, and Washington) have 
i been published. 
A table of the published folios is given in the list of Survey pub- 
lications. 
