THE HYDROGEOLOGY OF THE PLYMOUTH DISTRICT. 109 
The supply was easily exhausted previous to driving this heading. 
The level of the water has not been altered by the heading, 
although the supply and storage have been very much increased, 
and it now on an average supplies, say four days after rain, 400 
gallons per day. It does not fluctuate very violently with the 
rain-fall, and keeps up well in dry weather. All this points to 
an extended underground water-bearing fissure passing near this, 
and in the plane of cleavage of the rock. 
The results of gauging in December, 1876, were as follows : 
To surface of Water 
from ground. 
Rise. 
Galls. 
December 
7th. 
12' 0" 
>> 
8th. 
10 1 
I'll 
460 
>» 
9th. 
8 6 
V 7 
380 
>> 
10th. 
7 2 
14 
320 
>> 
11th. 
6 4 
0 10 
150 
12th. 
Eained 
5 7 
0 9" 
133 
5» 
13th. 
Rained 
4 10 
0 9" 
133 
At this last height, or a little above, the water flowed off in the top 
broken rock. The water does not come from the roof, but mostly 
from the sides, and from the bottom ; a considerable portion issues 
from the rock at the end of the heading. When excavating this 
the boring tools came on spar, and, judging from the surroundings, 
a rather extensive change of ground is close by. Thus it is held, 
that were the heading extended a short distance further, a very 
abundant supply would be had. 
We see here at a glance the relative value of the shaft and the 
heading in collecting water ; also the reason for the rule of the 
well-sinkers, " Always drive a heading north," or more generally, 
drive it against, and on the line of, the dip of the rock. Another 
maxim of theirs is, having sunk a well to water-bearing ground, 
gauge the quantity, and if it does not increase as the well is 
deepened, drive the heading as before. Rules may do very well 
for many operations ; but well-sinking of all others is the most 
dangerous for their successful application. Such a rule as this last 
might stop sinking at the very point of success. In this district, 
I have no doubt, many cases can be cited upholding these 
propositions, only great care must be had in applying them. 
Obviously for each case the details of the surrounding geology, as 
far as they affect the passage of water, are of the first im- 
