On Constructing Geological Models. 199 



ON CONSTEUCTING GEOLOGICAL MODELS. 



BY THE EEV. J. D. LA TOTJCHE. 



In many parts of the country the results of the physical dis- 

 turbances of the earth's surface are so strongly marked, that 

 an accurate representation of them by a model would afford 

 both interest and instruction. A geological map shows the 

 area covered by the different classes of formations, and by 

 means of the little arrows by which the Ordnance surveyors 

 indicate the dip of the rocks, may give a faint idea of the 

 physical features of the neighbourhood ; but to obtain a distinct 

 notion of the causes which have shaped the surface, have 

 elevated the hills, have depressed the valleys, of the primeval 

 streams which have denuded the whole ; the faults, the fissures, 

 the action of igneous matter and the other innumerable forces 

 which have played a part in the history of the earth's crust, 

 there is nothing so good as a model accurately constructed 

 according to scale, and coloured so as to show the various strata 

 and their inclination. A short account of how such a model 

 can be made may therefore be interesting, especially as the 

 means of doing so do not seem to be generally known ; at 

 least, when I first attempted to put my design of making one 

 into execution, I found it rather difficult to obtain such infor- 

 mation as would enable me to set to work. Indeed, the plans 

 which I have adopted may, though I have found them quite 

 effectual, be inferior to others in use, in which case I shall be 

 thankful to be better informed. 



The instrument indispeu sable in making the preliminary 

 survey for this model is a level of some kind. I have been 

 fortunate in obtaining the use of an excellent engineer's level 

 — an expensive instrument, costing some £12 or £15. A 

 drainer's level (the cost of which would be £1 10s.) would, 

 however, answer the purpose very well ; and first, since it is 

 necessary to keep this instrument constantly in adjustment, a 

 short account of how this is to be accomplished may be 

 desirable, taking for this purpose the more accurate of the 

 two kinds of level I have mentioned, since whatever corrections 

 are necessary for it, may be easily applied to the other. 



These corrections consist of two, the first being to secure 

 that the bubble should always rest in the centre of the glass 

 tube in whatever position it may be turned (for it revolves on 

 a vertical pivot); and the second, that when the telescope is 

 looked through, the bubble at the same time resting in the 

 middle of the tube, the hair which will be seen bisecting the 

 field of vision, may indicate an exact horizontal line. The 



