SURFACE FEATURES OF THE MOON — WRIGHT 175 



traced for approximately 1,500 miles; for this distance an initial 

 velocity of 1,480 meters (4,856 feet) per second is required and an 

 elevation angle of 26°. An initial velocity of ejection from terres- 

 trial volcanoes exceeding 2 kilometers a second has been deduced 

 from observations of the volcano Cotopaxi. It is evident, therefore, 

 that the ranges of ejection on the moon can easily have been pro- 

 duced by volcanic explosive forces comparable to those active on the 

 earth. On the moon the materials ejected from a lunar crater are 

 scattered far and wide, whereas on the earth the greater part of 

 the ejected rock fragments and blocks fall near and into the crater 

 orifice. As a result of this dispersion the lunar craters are cleaned 

 out as a rule and are of the nature of deep holes in the ground with 

 the floor of the crater below the level of the surrounding country; 

 the floors of terrestrial craters, on the other hand, are near the top 

 of the crater and high above the level of the adjacent country. This 

 is one of the factors to be taken into account in a study of lunar 

 craters. It is not permissible to conclude that, because the shape of 

 a lunar crater is similar to that of a terrestrial crater the mode of 

 formation of the two was the same. 



MAPPING THE MOON 



Before the geologist can make satisfactory progress in lunar 

 physiographic studies he must have a topographic map, at least of 

 the central portion, to aid him in visualizing the shapes of the lunar 

 surface features and of their relations one to another. He can then 

 classify the features, and by studying them in detail can acquire 

 a background of experience in lunar geology which is necessary to 

 competent interpretation of the phenomena observed. 



Of maps there are two kinds, the plan or base map and the topo- 

 graphic. Thus far, for the moon, only the first kind has been at- 

 tempted. It represents the moon's globular surface projected on a 

 definite plane and shows the features somewhat as the astronomer 

 sees them through his telescope. These maps have been drawn by 

 astronomers untrained in the principles of map-maldng, with the 

 result that existing maps are unsatisfactory in several respects; 

 the balance between map scale and amount of detail shown is not 

 realized and some of the maps are not easily legible; several lunar 

 maps have been prepared by men who were good observers, but 

 not good draughtsmen and unable to portray what they saw. In 

 other words, the existing maps suffer from the personal equations 

 of the men who drew them. Comparison of a lunar map made a 

 century ago with one made recently shows marked differences in the 

 representation of certain features ; on the basis of such a comparison 

 it has been concluded that changes have taken place here and there 



