Hilgendorf. — Artesian Wells in the Ghristchurch Area. 149 



20 ft. to the mile. At the time that the fall on the surface of the plains 

 between Rolleston and Lincoln was 68 ft. to the mile- — that is, when our 

 water-bearing stratum was deposited — the whole plain must have had a 

 much steeper gradient than at present. This would probably be due to 

 the much greater supply of waste to the above-gorge waters of the rivers, 

 so that in those times the present plains would have been much more like 

 the present-day shingle fans than like plains. That the gradient of the 

 plains was once much steeper than now is proved by the high terraces round 

 Woodstock, and by Racecourse Hill, a residual shingle mound some 60 ft. 

 high. The cutting into their beds of the present rivers is merely a continu- 

 ation of the process of lessening the gradient of the plain, the bed of the 

 Waimakariri being virtually level with the plains at their lower edge, and over 

 300 ft. below them at their upper limit. It is therefore in accord with what 

 I suppose would be the expectations of geologists that at one time the surface 

 of the plains should be much more steeply inclined than now, but that the 

 supply of waste should be so great as to form a deposit sloping nearly 70 ft. 

 to the mile forty miles away from the gorge is perhaps noteworthy.* 



It was stated above as evident that the collecting-ground for the. well 

 is nearer Lincoln than the nearest rainfall-station is. On the part of one 

 unacquainted with the country, a possible objection to this is that the 

 collecting-ground might equally well be more distant from Lincoln than 

 the farthest station is. The country between Springfield and the West 

 Coast, however, consists of mountains of greywackes and slates quite im- 

 pervious to water in large quantities, and, in any case, this water would 

 percolate out into the rivers flowing at the base of the mountains. The 

 amount of water in the Waimakariri is, moreover, a gauge of the amount 

 of rain falling on these mountains, and I have been so fortunate as to be 

 supplied with daily readings of the height of the river during several months. 

 Most of the floods that my records show occurred nearly contemporaneously 

 with considerable rainfalls on the plains, and the subsequent rises of the 

 well could not, therefore, be stated as dependent on, or independent of, 

 the rises in the river. On the 23rd March, however (see the arrow-head in 

 fig. 5), there was a heavy flood, sufficient to stop the mails at the Bealey, 

 but, as fig. 5 shows, there was no sign of any rise or arrest of the decline 

 of the well until rain fell in the second week after the flood. 



Although it is impossible that the rain falling on the mountains shoidd 

 directly find its way into the water-bearing stratum tapped by the well, it 

 seemed quite possible that after reaching the river the water might percolate 

 into such a stratum where the river runs across its outcrop. This, indeed, is 

 probably the common opinion held ; but the observations made do not sup- 

 port the supposition, as far as the well at Lincoln goes. The observations on 



* At the meeting at which this paper was read Mr. Speight pointed out that the 

 conclusion reached here is probably incorrect. His observations on the strata pierced 

 by wells near Christchurch shows that the deep-lying strata are at practically the 

 same slope as the present surface of the plana. It is a matter of common observation 



that clay strata, though common near Christ- 

 / / ~ S N church, disappear farther up the plains, and it 



4, • y N^ it is probable that Rolleston marks the distance 



from Lincoln at which the clay stratum over the 

 water- bear ing stratum fades away, rather than the 

 outcrop of a series of strata regular in thickness 

 from the base of the well to the outcrop. This 

 idea is shown in the accompanying diagrammatic 

 sketch, where the arrow-head .shows the position of Rolleston. Mr. Speight's in- 

 terpretation of the facts seems to me correct, and invalidates the conclusions above 

 drawn as to the former slope of the surface of the plains. 



