﻿240 EEV. W. HOWCHIN ON GLACIA.L BEDS OF [May I908, 



(b) The Erratics. 



The occurrence of isolated and irregularly-distributed boulders is 

 a constant feature in the exposures of the gritty mudstone, or till; 

 but these stones vary in size, relative numbers, and to some extent 

 in their petrological types, in different localities. A close-grained 

 and very siliceous quartzite usually supplies the commonest variety. 

 These quartzite-boulders are clearly distingaishable, even macro- 

 scopically, from the bedded quartzites which are sometimes present 

 in the till, and also from the quartzites which occupy an inferior 

 position to the glacial beds in the Cambrian series. ^ 



A rough attempt has been made, in several localities, to estimate 

 the relative numbers of the various kinds of stones occurring as 

 erratics. The following are given as examples of this attempt at 

 grouping, and in each case the various classes of stones are mentioned 

 in the order of their frequency of occurrence. 



(1) Petersburg Ranges, 160 miles north of Adelaide. 

 — Quartzite, gneiss, dark porphyry, coarse-grained granite, schor- 

 laceous quartz, pink porphyry, rocks from basic dykes, graphic 

 granite, mica-schist, and siliceous limestone. 



(2) Another locality in the Petersburg Eanges.-^ 

 Light-coloured quartzites, fine-grained whitish granite, aplite, 

 chloritic gneiss, mica-schist, gneissic granite, quartz-porphyry, 

 coarse granite (with pink orthoclase), pegmatite, quartz-felsite, 

 coarse white granite, and garnet-gneiss. 



(3) Jamestown, 150 miles north-west of Adelaide. — 

 Here the boulder-beds are decomposed, and form excellent soil for 

 wheat-growing. The farmers have cleared their land of the larger 

 boulders by harnessing horses to them, and dragging them to the 

 fences. In some places the stones are too numerous and large for 

 this method of clearance, and plots are left uncultivated. In one 

 of these the following erratics were noted in their order of fre- 

 quency : — Gneiss of various kinds, including some exhibiting augen- 

 structure ; white granite, with porphyritic crystals of orthoclase ; 

 quartzite, aplite, biotite-schist, muscovite-schist, dark porphyry, 

 pegmatite, pink granite, and white marble. A quartz-porphyry 

 erratic in this group measured 9 feet 6 inches in length ; and a 

 large quartzite-boulder near it exhibited a fine glacial polish, as 

 also striae. 



(4) The Burra, 100 miles north of Adelaide. — The 

 erratics in this locality are, as a rule, relatively smaller and fewer 

 in number than usual. The following were the largest noted : — 

 Quartzites, up to 12 inches ; porphyritic rock, 15 inches ; mica- 

 schist, 2 feet ; aplite, 2 feet 9 inches. 



None of the boulders found in the till are of local 

 origin. Some are of well-known types which are characteristic 



^ See W. G. Woolnough, ' Petrographical Notes on some South Australian 

 Quartzites, Sandstones, & Belated- Rocks ' Trans. Key. Soe. S. Austral, vol. xxviii 

 (1904) p. 207. 



