INTRODUCTION 93 



in which the study is carried on. Sometimes one branch and 

 sometimes another will afford locally special facilities. But there 

 are few districts in which some branch cannot be pursued with 

 exceptional success, and it is in discovering the most appropriate 

 branch that the skill, knowledge, and insight of the teacher is 

 called out. Sea shores, hill country, abrupt valleys, broad plains, 

 lakes, alluvial lands, glaciated country, bare rocks, forest land, 

 mining ground, each presents its own peculiar problems and 

 each one cannot fail to furnish material for observation and 

 induction which in the hands of a skilled teacher will lead to 

 important results, not only in earth knowledge itself, but in giving 

 a better understanding of the physical environment of life. 



The destruction of the surface of the country by some form 

 or other of denudation is a process which can be studied anywhere. 

 Facts can be easily collected and reasoned upon, and certain 

 inevitable consequences so readily foreseen that this branch of 

 study seems to be naturally marked out as the most convenient 

 and logical starting-point, and the one most readily available for 

 all teachers of the subject. 



Sir Archibald Geikie, in his admirable Primer of Geology, 

 made good use of the phenomena of a road as an introduction 

 to the study of denudation, and the condition of most of the 

 roads and lanes in the United Kingdom still leaves them quite 

 as suitable for physiographic study as for bearing the strain of 

 modern traffic. 



A road surface may be regarded as made up of stones, rounded 

 or edged, packed fairly close together, the remaining interstices 

 being filled with smaller fragments, grit, and dust. If this 

 material is packed tightly together, if it is thick enough and rests 

 on a solid foundation, and if the weather is not too wet, too dry, 

 or too cold, the coating will remain unbroken, and will stand 

 the passage of traffic over it. The friction of wheels will, however, 

 begin to wear away the surface of the stones, and the weight 

 of traffic will tend to crush down any irregular projections of 

 them and any softer constituents that they may contain. Both 

 these causes will be productive of finely crushed rock, which 

 is known as dust when dry and as mud when wet. The change 

 of solid, hard stone into dust or mud is known to geologists as 



