262 Dr. T. Stacey Wilson — The Making of Geological Models. 



Thus a model of the floor of the district is obtained with the 

 gentle rise and fall of its folds and the abrupt drop of its faults. 



It is quite evident that a similar base might be built up with 

 cardboard or wax sheets, or modelled in clay, or cast in plaster^ 

 or obtained in a less laborious way than the carving of a wood 

 block. The only essentials are rigidity and accuracy. 



5. On this carved surface the successive geological formations 

 were built up to scale by means of layers of differently coloured 

 paraffin wax or waxed felt of the proper thickness. 



6. In order to ensure the first layer being of the proper thickness- 

 the following method is employed. Pins with small heads 

 (entomological pins) a little longer than the thickness of the 

 stratum (or cut to the requisite length) were driven in all over 

 the wooden base. A convenient way of securing the right height 

 was to take a strong metal pencil-case, remove the lead, and draw 

 back the stop till the lead chamber was just the required depth. 

 On inserting the pins' heads into the lead chamber the peucil-cas© 

 was used to drive the pins in exactly the right distance with the 

 minimum expenditure of time and trouble. 



7. The wood is next thoroughly wetted to prevent the paraffin 

 sticking to it, or it may be covered with wet tissue paper, and 

 the coloured wax is spread over the model to the level of tho 

 pin heads. Small strips of wet stiff paper or pieces of tin should 

 be inserted along the fault planes so as to give sharpness to the 

 edges of the strata there. 



8. As soon as the first layer sets the pins are removed and more 

 pins inserted in the same way to give the thickness of the next 

 stratum of coloured wax. To strengthen the model some of the 

 layers should be put on with waxed cloth or felt in sheets, but 

 this, though desirable, is not absolutely necessary. 



9. Where a bed crops out on the surface the coloured wax is 

 carried a little beyond the area occupied by the bed, as shown on 

 the geological map. 



10. When all the layers have been put on and have set fairly firm 

 the surface is modelled with the gouge and penknife so as to 

 show the hills and valleys. If this could be done quite accurately,^ 

 and if the structure and thicknesses were quite correctly rendered, 

 it is evident that the surface of the model should exactly 

 correspond with the country. In practice, however, both the known 

 contour of the country and the known outcrops of the beds are 

 utilised for making the surface of the model approximately accurate. 



Before beginning the modelling of the base of a country it is 

 best to indicate by long pins the position of the chief landmarks 

 in the area dealt with. These can be maintained in position as 

 guides throughout the whole process, and removed when the model 

 is finished. 



Slight modifications of this process which might easily be devised 

 will obviously render it applicable to problems related to intrusive 

 and volcanic rocks. 



