DETERMINING THE STRUCTURE OF BEDDED ROCKS 163 
moval of some of the filler in others the surface soon shapes itself 
into one of broad curves simulating those of the average deformed 
beds. 
There are many short cuts in the technique of model making 
with which the worker will become familiar after a little practice. 
The relation of the vertical to the horizontal scale makes no 
essential difference for if the work is accurately done the resulting 
contours will always be the same. Obviously, the model must 
be contoured with the same vertical scale as that used in its 
construction and the contours must be reckoned and numbered 
in reference to the assumed elevation of the wood base. The 
writer has found that on the average a vertical scale which 
gives a relief of about 3 inches to a model with a base of 16 
inches is very desirable. This makes convenient the use of a 
horizontal scale of 2 or 4 inches to the mile, the usual scales 
used in plane table work. The models of this size or in units of 
this size permit the representation of 16 or more square miles, 
about the average involved in the report of the consulting 
petroleum geologist. 
It is often desirable to preserve the model in the form of a 
plaster cast. In making casts, care should be taken to preserve 
the proper relations between the horizontal reference plane and 
the surface representing the structure. Perhaps the most simple 
way of accomplishing this is to level the base and then make the 
negative by pouring plaster of paris over the model (which has 
been confined by sides of suitable height) till the plaster entirely 
covers the sand and in its liquid state forms a level surface. 
When the negative is reversed, insulated, and confined within a 
suitable frame, the positive may be poured and if the lower side 
of the reversed negative is horizontal the liquid plaster of the 
positive will form a flat surface with which the structural surface 
will have the proper attitude. It has been found that for most 
work a thin soft soap makes the best insulation between nega- 
tive and cast. 
For contouring the writer has found very convenient an appa- 
ratus such as that shown in figure 6. A less elaborate outlay 
answers the purpose well and is, perhaps, even more accurate. 
