
GEOLOGICAL SURVEYING 289 
those beds are interstratified. An adequate detailed acquaint- 
-ance with the whole series of rocks occurring in a district 
often enables the observer to locate the geological horizon of 
isolated rock-exposures, and to plot the position of boundary- 
lines with wonderful accuracy, even in places where the 
ground is thickly mantled with superficial deposits. 
(f) Zransported Detritus in Stream-courses.—In cases where 
the geological] position of the rocks exposed in a stream is not 
suggested by the character of the rocks themselves, the field- 
geologist does well to examine carefully the gravel and 
G@eniis, as he proceeds up the valley. Should- he ‘detect 
fragments of a rock, say, limestone, which he has already 
encountered zz szfw elsewhere in the same district, he makes 
careful note of his find and continues to follow the spoor up- 
stream. Possibly the limestone fragments become more and 
more numerous as he goes on his way, while, at the same 
time, they are less water-worn, and occasionally, perhaps 
attain a relatively large size. Eventually, at some particular 
spot they cease to occur—the obvious inference from which is 
that the limestone itself must crop out here or at some short 
distance up-stream. Ina case of this kind a geologist would 
Heamiraiy seek to strengthen the evidence by carefully 
examining the adjacent valley-slopes for similar angular 
fragments. 
After direct and indirect evidence of every kind has been 
exhausted, we probably find that there are still certain spaces 
upon our map across which boundary-lines cannot be traced. 
Wide sheets of peat or alluvium, for example, may effectually 
conceal broad areas. Should the map we are using be on a 
large scale, say 6 inches to the mile, we should stop the lines 
abruptly where they meet the obscuring sheet of alluvium or 
peat, and colour the latter as a separate formation. On small- 
scale maps, however, it may be desirable in many cases to 
catry a line—more especially if it be the outcrop of some 
important or valuable seam—across areas which are covered 
by peat or alluvium. This may be safely done when we have 
assured ourselves that there is no interruption or break in the 
continuity of the strata. When the conditions are such as 
represented in Fig. 111, there can be no doubt that the out- 
crop of the limestone (2) must continue across the area cons 
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