THE NATURE AND ORIGIN OF GILGAI COUNTRY. 



357 



mode of origin. It is suggested that in the gilgai zone of 

 the Pilliga Scrub this type of country was a lake without 

 outlet at no distant geological period. In this period the 

 rainfall of our north-western districts was exceeding low, 

 and the gilgai lake was dry at intervals, receiving only 

 small streams from mudsprings in the hilly country twenty 

 miles away. 



The contraction of the clay deposits on drying, the 

 growth of tufty grasses in the salt marshes and chemical 

 precipitation on the tussocks gave rise to a hummocky 

 surface. When moister conditions revived the creeks from 

 the Warrumbungle Mountains integrated drainage was 

 restored. The inequalities of surface on the gilgai area 

 were accentuated by capillarity when new conditions had 

 been established. 



Various facts of physiographic interest pointing to grave 

 -climatic changes in recent and late Tertiary times are 

 recorded in this paper. 



Analysis of Gilgai Soils. 



Table A. — Mechanical Analyses. 



No. 



Colour. 



Reaction. 



Water 

 Capacity. 



Capillarity. 

 Inches in three 



Gravel 



Sand. 



Clay. 









Per cent. 



hours. 



Per cent. 



Per cent- 



Per cent. 



4 



black 



strongly alkaline 



good 49 



fair 3 



102 



183 



71-5 



5 



>t 



faintly acid 



fair 44 



fair 4 



7-4 



107 



81-4 



6 



t> 



alkaline 



good 47 



fair 3 



10-7 



19-3 



70-0 



9 



>> 



alkaline 



good 47 



fair 4-J 



10-0 



223 



66-9 



11 



„ 



strongly alkaline 



high 63 



poor 2 



2-4 



l'O 



966 



15 



red 



very faintly acid 



low 33 



excellent 10 



16-4 



31-7 



519 







Table B.— 



Chemical Analyses. 





No. 



Moisture. 



Volatile. 



Nitrogen. 



N. 



Lime. 

 CaO 



Potash. 

 K 2 



Phosphoric 

 Acid. 

 P S 5 



Manganese 

 Oxide 

 Mn 3 4 





Per cent. 



Per cent. 



Per cent. 



Per cent. 



Per cent. 



Per cent. 



Per cent. 



4 



6-52 



6-67 



•042 



1-080 



•215 



•152 



•085 



5 



6-58 



9-86 



•142 



•446 



•225 



•188 



155 



6 



755 



7-27 



•070 



•600 



•335 



•128 



•100 



9 



350 



4-50 



042 



•308 



•087 



•049 



•120 



11 



6-29 



521 



014 



•712 



•126 



•079 



•062 



15 



1-94 



501 



112 



•260 



175 



•113 



•270 



