622 Methods of Instruction in General Geology. (July 
effective in imparting the information desired than the more 
finished ready-made diagrams. The value of the blackboard 
sketch is in the fact that it emphasizes your words, makes them 
more vivid and expressive; with crayon in hand as you talk, thi f 
lines of the sketch become a kind of lineal gesture. l 
This result is particularly seen when the relations of two or 
more objects are expressed, or when serial or gradual changes or 
developments are -considered, as in explaining Darwin’s theory 
of the formation of coral reefs and islands, or the structure and 
growth of a common volcanic cone, or the effects of erosion in 
_ cutting through a series of hard and soft stratified rocks, and the 
relation of dykes to eruptive sheets, to volcanic lava-beds or 
fissure lava-fields. 
All such phenomena are more vividly expressed to the 
ginner by blackboard sketches than by the more perfect dia- 
gram, and the reasons seem to lie in the fact that the student 
out variations and the relations of one form to another, I fi 
the blackboard a great assistance. : 
_ A method specially successful of late in the teaching 0 
geology, The method I refer to may be called the exhausti 
study of types, and such a book as Huxley’s “Crayfish” m 
illustrate its application. 
familiar and typical example of all the principles involved in its 
structure, function, or other relations, Afterwards, the wideni 
of one’s knowledge becomes a simple and also a systematt 
mental noting of likenesses and differences. The knowled 
thus acquired, instead of being vague and general, becom 
positive and precise. “ 
In applying this method to a geology, l select a fe 
