iii6 PRINCIPLES OF STRATIGRAPHY 



formation map are also known. The best examples are the com- 

 plete maps of New York State, on the scale of 5 miles to i inch, 

 while the map issued with the summary final report of the Second 

 Geological Survey of Pennsylvania may serve as another example. 

 These maps represent series rather than formations, though in 

 many cases the series consist practically of one formation only, such 

 as the Onondaga limestone. In other cases the unit mapped in- 

 cludes what, on a map of larger scale, would be represented as 

 several distinct formations. Such, for example, is the Clinton series 

 which on the New York State map is shown by one color only, 

 while on the map of the Rochester quadrangle it is shown as five 

 distinct formations. The Portage group, represented as a single 

 unit on the State map, is divided into eleven formations on the 

 Canandaigua-Naples map (1/62,500), exclusive of the Genesee 

 shale and the Tully limestone. 



Notation of Formations on Map. In addition to the color and 

 pattern used in the representation of the formations or larger 

 units, a conventional sign, which may be a letter, or combination 

 of letters, or a number is used. This insures greater ease in identi- 

 fying the formation on the map. The United States Survey, 

 in its folios, has adopted a group of letters as the symbol, the 

 first letter representing the horizon, the other the name of the 

 formation. Thus, on the Hancock quadrangle, where the 

 Siluric is represented by five formations some of the symbols 

 are : Sc, Clinton shale; Smk, McKenzie formation; Stw, Tonolo- 

 zvay limestone. The S in each case signifies Siluric (Silurian). 

 In the Geological Map of North America, above referred to, 

 numbers are used to further differentiate the systems from one 

 another. 



Legend. In order that the proper superposition of the forma- 

 tion may be ascertained, a legend is added consisting of small 

 rectangles colored to correspond to the color pattern which it repre- 

 sents on the map, and furnished, moreover, with the corresponding 

 symbol and the name of the formation. These rectangles are ar- 

 ranged in the order of superposition or sequence. As a rule, the 

 oldest formation is put at the bottom and the youngest on top. 

 The New York State Survey has, however, adopted in some of its 

 larger maps the reverse arrangement, the oldest being on the top. 

 This is done, apparently, to bring the color pattern of the legend 

 into harmony with that of the map ; in which the successive older 

 formations crop out in belts of decreasing age from the north 

 southward. 



