196 ARTESIAN WELLS IN DORSET AND ELSEWHERE. 



dip from 1-5 to 2 southwards, taking a sort of average, 

 although there may be subsidiary folds within the general 

 syncline. This is in conformity with the general rule that the 

 dip of the northern limb is at a lower angle than that of the 

 southern one. 



From the Bovington Borehole to the margin of the Chalk 

 escarpment near Bulbarrow is a distance of about 12 miles in a 

 northerly direction, and if we deduct 3 miles for areas covered by 

 Tertiaries along this line, there remains 9 miles of Chalk outcrop 

 on which the rain may fall directly to feed its underground waters. 

 It is well known that a large percentage of the water which 

 falls on this absorbent formation sinks into it instead of flowing 

 over the surface as is the case with clays. Moreover, the rain- 

 fall on the North Dorset downs is much heavier than in the 

 Frome valley, and still more so in comparison with Weymouth. 

 Hence there is an abundant supply in the region lying to the 

 north of the Borehole, and assuming the preponderance of a 

 southerly dip, which may be taken for granted, this water in its 

 underground passage is bound to find its way south until its 

 progress is arrested by the pressure of water from the opposite 

 limb of the syncline. Meanwhile it is kept down by Tertiary 

 Beds, &c., having a thickness of 28oft. Taking the average 

 elevation of the base of the Chalk between the Dorsetshire Gap 

 and Bulbarrow at 65 oft. above O.D., this may be accepted as the 

 elevation at the outcrop in the escarpment of the North Dorset 

 Downs. On the supposition that the Chalk is gooft. thick 

 beneath Bovington, this would bring the base of the Chalk to 

 i, 01 oft. below O.D. at the Borehole. Adding these two sums 

 together, we obtain i,66oft. as the difference in height of the 

 base of the Chalk between the one point and the other. This 

 sum of i,66oft. vertical has to be distributed over a horizontal 

 distance of 12 miles, and this shows an incline of i in 38 = 

 about 1*5. Thus from the results of an independent calcula- 

 tion, we obtain precisely the same amount of dip for the 

 northern limb of the syncline as had previously been deduced in 

 another way. (See page 189.) 



