PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 7 



More rarely it occurs massive in deposits of con- 

 siderable extent and is mined for commercial 



purp 



oses. 



In the rock collection the samples are as a rule trimmed with a ham- 

 mer into sizes approximately 3£ by 4£ by 1 inch, this form liavin*^ 

 been found most convenient when everything- is taken into considera- 

 tion. This rule is not, however, inviolable, and both size and shape are 

 allowed to vary when the character of the rock necessitates or renders 

 this advisable. Care is taken in all cases to procure so far as possible 

 fresh and characteristic materials and that no specimen shall show 

 abrasive marks from the hammer or other agencies on its exposed sur- 

 face. 



Advantage has here been taken of an opportunity to bring together 

 as large a series as the present facilities will allow of such rocks as 

 have been the subject of the close methods of scrutiny adopted in mod- 

 ern petrography. It therefore happens that certain groups are repre- 

 sented in greater profusion than their geological importance seemingly 

 warrants. The system of installation is, however, by no means inelas- 

 tic, and when these other groups shall in their turn receive the attention 

 they merit a place can readily be made for them by substitution, or 

 better yet by an expansion of the entire series. 



In the arrangement of the exhibition portion of this collection the 

 curator is at once confronted with one of the most unsatisfactory prob- 

 lems in modern petrology, that relating to classification. The rapid 

 strides which this branch of the science is now making render any 

 system likely to be adopted of only provisional value, and what is 

 written today may by the time it appears in print be so far out of date 

 as to be more than unsatisfactory. However readily one may classify 

 a series designed for study only, he is, with his exhibition series, at 

 once reminded that each specimen, selected with a definite purpose in 

 view, with an eye to geographical as well as geological distribution, 

 must be called by a definite name, and placed in a definite position in 

 the series. 



Whatever mental reservations the curator may have can not be made 

 to appear on the label, nor, indeed, is it desirable that they should. 

 Every teacher must long have recognized the fact that in the beginning 

 no student should be confronted with all the uncertainties of any 

 problem. Such a course tends only to confuse and discourage. It is 

 best at first to treat of matters as apparent certainties, and when the 

 student shall have sufficiently progressed the uncertainties will grad- 

 ually unfold of themselves. 



So in arranging this petrographie collection the curator has, in his 

 classification, indicated a simplicity which perhaps docs not actually 

 exist, a system which aimed to be not so complicated as to confuse the 



