79° STUDIES FOR STUDENTS 



nels, and government dredgings afford a body of knowledge 

 of the submarine bed rock equal to that obtainable from most 

 land areas. 



The necessity for drawing boundaries in covered areas has 

 evolved a body of largely unwritten doctrine which may not 

 inaptly be referred to as the canons of geological mapping 

 for the crystalline areas. Some of these canons are truths, 

 others half truths, and still others have little basis in fact. 

 Some of the more important of them are subjoined with brief 

 critical notes. They are of course to be weighed in connection 

 with one another and a compromise decided upon. 



1. Propinquity- of outcrop. — Near-lying outcrops should be con- 

 nected when not separated by outcrops of a different formation. 

 This rule is almost axiomatic and thoroughly sound in principle 

 provided no contrary evidence is at hand; but there is yet a possi- 

 bility of error even when outcrops are separated by less than 

 one hundred feet. 



2. Contacts covered. — The boimdary is not at the margin of 

 either of the separated outcrops of different formations , but at some 

 line between. This is a general rule but with many exceptions. 



3. Strike. — Direction of boundary conforms to strike of near 

 outcrops. A rule of much value but to be weighed with other 

 considerations before decision is made. 



4. Slopes. — On slopes boundaries should be corrected for strike 

 and dip. With vertical beds if the strike is away from the ele- 

 vation in the general direction in which the boundary is being 

 extended, the boundary will run down the slope, if toward the 

 elevation it will run up the slope. If the slope also changes in 

 such manner that the contour line converges less or diverges 

 more from the strike the boundary will curve more rapidly 

 down the slope; if the contours diverge less from or converge 

 more with the strike in that same direction, the boundaries will 

 the more rapidly ascend the slope. If the dip is with the ele- 

 vation the boundary bends the more up the slope, if out from the 

 elevation the more down the slope. 



5. Topographic breaks. — Sudden changes i?i slope not caused 

 by talus or drift are likely to correspond to contacts. 



