Page:Popular Science Monthly Volume 10.djvu/733

Rh refer here to the use of variously-colored pencils for expressing at once, upon map or note-book, the different rock-masses which may occur in a district. Water-colors are, of course, ultimately employed for representing the geological formations on the finished map. But a few bits of colored pencils carried in his pocket save the geologist much needless writing in the field. To a red dot or line he attaches a particular meaning, and he places it on his map without further explanation than the local peculiarities of the place may require.

This leads me to remark, also, that he necessarily adopts a system of signs and contractions on his map, not only to save writing, but to prevent the map from being so overcrowded with notes as to become hopelessly confused. Every field-geologist insensibly adopts contractions of his own. For the fundamental facts of geological structure, however, it is eminently desirable that the same signs and symbols should be used with the same meaning on all published geological maps. The subjoined diagram (Fig. 3) shows some of the signs used on the maps of the Geological Survey of Great Britain and Ireland.

Such are the few prime instruments required in field-geology. We may add others from time to time, according to the nature of the work, which in each region will naturally suggest the changes that may be most advantageously made. A small bottle of weak hydrochloric acid, carried in a protecting wooden box, or case, is sometimes of use in testing for carbonates, particularly in regions where rocks of different characters come to resemble each other on their weathered surfaces. When Sir William Logan was carrying on the survey of the Laurentian limestones of Canada, he received much help from what he called his "limestone spear." This was a sharp-pointed bit of iron fixed to the end of a pole or a walking stick. He enlisted farmers and others in his operations, instructed them in the use of the spear, and obtained information which gave him a good general notion of the distribution of the limestone. The spear was thrust down through the soil until it struck the rock below. It was then pulled up, and the powder of stone adhering to the iron point was tested with acid. If, after trying a number of places all round, the observer uniformly obtained a brisk effervescence when the acid drop fell on the point of his spear, he inferred that the solid limestone existed below, and noted the fact on his map accordingly.

When the Geological Survey was busy with the great Wealden area of the southeast of England, my colleagues used what they