362 FRA NK B URSLE Y TA YL OR 



I have extended them beyond these features by interpolation 

 for a short distance, which may be defined as interpolation to 

 the first degree. By this I mean that degree or amount of inter- 

 polation which any cautious and experienced observer would 

 make without hesitation. Geologists who study the indurated 

 rocks, especially in a drift-covered region, are continually driven 

 to interpolate between outcrops. Where the distance is not 

 great and the relations appear to be simple, such interpolation is 

 regarded as a matter of no great difficulty, seldom involving 

 serious risk of error, and requiring no very critical weighing of 

 the phenomena for its application. It is like interpolating the 

 course of a brook through a wood lot or a thicket when the 

 places of its entrance and exit are known and the general rela- 

 tions are seen to be simple. Of course, many small errors will 

 inevitably arise from such interpolation, but they seldom affect 

 the larger elements of the problem in hand. The first degree of 

 interpolation goes but a little way beyond what we can actually 

 see. Where the restoration of the ice-borders in Fig. 10 has 

 been accomplished by the use of interpolation which involves 

 more uncertainty than the first degree, as here defined, I have 

 drawn them as broken lines. Of course, the limit of the first 

 degree of interpolation is an arbitrary one, but I have endeav 

 ored to keep on the safe side. The higher mountain areas, like 

 Mount Washington, showed very few recognizable ice-border 

 features and the course of the ice-borders across these areas is 

 drawn almost wholly by interpolation. In general, therefore, 

 the margin of error is much larger here than in the lower areas. 

 Nevertheless, an effort was made to minimize such errors by 

 carrying the studies over all the contiguous low ground, even 

 where some of this extended outside of the quadrangles. 



CONCLUSION. 



If these studies have been guided by right methods and the 

 interpretations made on a foundation of correct principles, we 

 seem justified in concluding that the ice-border which retreated 

 to the northwest across Berkshire county was the eastern edge 

 or limb of the Hudson valley lobe ; that the positions occupied 



