186 R. LOGAN JACK, LL.I)., F.G.S., ETC.^ ON 



mimber of bores have been made over a series of years, 

 speculation on the subject is hazardous.* The pressiu'e of 

 some bores has diminished and a^ain increased, ^ome bores 

 have ceased — perhaps temporarily — to flow over the surface. 

 In some cases a diminished flow has been traced to wearing 

 out of the tubing and consequent caving in. In others, the 

 pipes have been more or less fllled up with a mineral deposit. 

 But in the great majority of cases there has been no failure 

 in the flow. 



6. Geological Conditions. — The greater part of the Avestern 

 interior of Queensland is composed of soft strata of Lower 

 Cretaceous age, consisting of clay shales, limestones, and 

 sandstones. These strata are so disposed that the lower 

 members of the series crop out on the western flanks of the 

 coast range, where not only is the elevation of the surface 

 greater than in the downs to the west, but where also the 

 rainfall is comparatively abundant. In the year 1894 I went 

 out, accompanied by Mr. A. Gibb Maitland, now Government 

 Geologist of Western Australia, with the view of delimiting 

 the artesian-water-bearing area, and collecting information 

 regarding the lower members of the series of rocks in which 

 the water is found. We observed all along the eastern 

 margin of the Cretaceous area a great thickness of an ex- 

 ceedhigly porous sandstone so incoherent that when saturated 

 with water a piece of it would crumble instantly into sand. 

 To this rock we gave the name of the " Blythesdale Bray- 

 stone." Owing to the low dip, the outcrop of this ideally 

 permeable stratum occupies a belt varying from five to 70 

 miles in width, but the " Braystone " finally disappears 

 beneath the argillaceous and calcareous upper members of 

 the series which form the soil of the downs to the west. 

 Roughly speaking, there is a mean annual rainfall of 27 

 inches in the regions where the "Braystone'' comes to the 

 surface. Several great rivers, such as the Flinders, com- 

 mence their career as running streams of considerable volume, 

 but, except in wet seasons, disappear while crossing the 

 outcrop of the " Braystone," and the Avater must be carried, 

 with the permeable stratum, beneath the clay-shales of the 

 downs. The records of bores shoAv, as a rule, that the 



* 111 Lancashire and Cheshire the effects of dry and wet seasons do 

 not show themselves in the deep wells of the new red sandstone till 

 about six months after their occurrence, this period being required for 

 percolation. — Ed. 



