DEMONSTRATION MATERIAL IN GEOLOGY 85 



Such an exercise is, even after hearing a lecture on the origin of coral 

 reefs, fully worthy of the most earnest graduate student; and, moreover, 

 until such an exercise is worked out as rigorously as if it were a geometrical 

 theorem, even the most earnest graduate will not have mastered the coral-reef 

 problem. In my opinion, it is chiefly because good models make it possible 

 to introduce serious problem work into geological teaching that they are to 

 be recommended. 



Any model that is not susceptible of use as the basis of a laboratory prob- 

 lem is hardly worth its cost, unless the cost is very low. On the other hand, 

 even if susceptible of such use, a single model is not ordinarily worth a very 

 high cost, because the payment of a high cost for one model means that many 

 other models are excluded. The model of Borabora above referred to is a 

 magnificient adornment of a museum; but, in size as weir as in cost, it far 

 exceeds the limits that can ordinarily be allowed for geological teaching. A 

 smaller, simplier, and cheaper model might serve very well as the basis of 

 the two exercises above outlined ; and then, for educational purposes, the cost 

 of one large and very expensive model could be divided among many simpler 

 but very useful models. 



The model illustrating two faults in horizontal strata, shown by Professor 

 Cleland in a lantern slide, is more useful in its geometrical than in its geo- 

 logical relations, for it tends to perpetuate an obsolescent geological habit of 

 studying underground structure while neglecting surface form. Such a model 

 may serve for a mining engineer, but it is not enough for a geologist. If used 

 as shown, it should be supplemented by laboratory exercises in which the 

 student has to draw sections and maps illustrating various stages of surface 

 erosion on the faulted mass, and these stages should be extended past the 

 first cycle of erosion on the faulted mass, at the end of which the baseleveled 

 fault has lost its control of surface relief and is expressed only by a differ- 

 ence of soil on the two sides of its trace ; and the later stages should include, 

 at least for advanced students, those of a second cycle of erosion after uplift 

 without renewed faulting, when the fault will be topographically redeveloped 

 in a fault-line valley or in a resequent or an obsequent fault-line scarp ; and 

 the various striking differences between such fault-line features and true 

 fault-scarps should be carefully written out. Without such extension of the 

 exercise, the model is not used to its full value and the subject of faults is 

 not treated with a sufficiently serious discipline. If the model be constructed 

 so that surface parts may be successively removed to illustrate various stages 

 of erosion in a first and a second cycle after faulting, so much the better. 



As to dip and strike, I am persuaded that if there is any difficulty whatever 

 in the teaching of such simple matters, it is due either to ignorance of ge- 

 ometry on the part of the student or to inexperience in explanation on the 

 part of the teacher. In the latter case it may be that the teacher expects 

 the student to attach ideas to the words "dip" and "strike." Such a pro- 

 cedure nearly always results in more or less mental confusion on the student's 

 part. On the other hand, if the words "dip" and "strike" are not mentioned, 

 but instead the facts of occurrence regarding tilted rocks are simply described 

 in lectures, or illustrated in the laboratory, or studied in the field, until they 

 become so familiar that the rock attitudes are easily talked about by the 

 student in words of his own choosing, then he may replace his own words 

 by dip and strike without any mental difficulty. 



