Academy of Sciences] 



No. 1] 



AUSTRALASIA. 



39 



basalts that overlie the Carboniferous formations and perhaps extended up into the Permo- 

 Carboniferous. The investigations of Mr. W. R. Browne and the writer ('20) show that the 

 volcanic rocks, sills, and dikes of the Carboniferous series comprise normally calcic rocks such 

 as andesites, dolerites, and basalts with rather more alkaline types such as soda-rhyolite, kera- 

 tophyre, and an albitized dolerite. The distinction between the Devonian and Carboniferous 

 types of eruptive rocks in New South Wales is therefore comparable with that between the 

 Devonian and Carboniferous types in the Urals (p. 16). As in the Urals, the changes of 

 eruptive rock-facies marked the change from geosynclinal conditions of crust-sagging to orogenic 

 conditions of up-folding of the crust. This produced an unconformity above Carboniferous beds 

 which, though marked in the northern part of this serpentine-belt where the lower and middle 

 portion of the Permo-Carboniferous series is absent, is much less marked near Maitland, where 

 the complete Permo-Carboniferous sequence rests with disconformity or slight unconformity 

 upon the Carboniferous beds (David '19). This Carboniferous folding, ushered in by the out- 

 pouring of lavas and production of sills, culminated in the development of the plutonc series of 

 New England which commenced with the intrusion of peridotites, intimately associated with the 

 slightly newer gabbros, which were followed by a long series of granites, the later members of 

 which invaded even Permo-Carboniferous marine beds that had developed during a lull in the 

 orogeny (Andrews '05). 



Western 

 Floms 



England 



Plateau 



Flo. 11.— Geological section across the Great Serpentine Belt of New South Wales. (See fig. 10.) 



1. Highly folded Devonian sediments. 



2. Middle Devonian agglomerate. 



3. Upper Devonian mudstone. 



4. Lower Carboniferous mudstone. 



5. Middle and Upper Carboniferous conglomerate, etc. 



6. Serpentine. 



7. Granite. 



8. Permian. 



9. Triassic. 



10. Tertiary trachyte, etc. 



The characteristic feature of this occurrence of peridotite (now serpentine) is its linear 

 character. It runs intermittently for over 250 miles from the coast, a short distance north of 

 Port Stephens, to Warialda. (See Benson '17, '18a.) The serpentine-belt is rarely as much 

 as a mile in width; frequently it is only a few yards across. Generally it occupies a well-marked 

 structure-plane separating highly crushed from less crushed rock. (See figs. 10, 11.) The fact 

 that the boundaries of the subsequent intrusions of granite are so highly irregular, and their 

 irregularities in no way affect the continuity of these lines of serpentine shows most strikingly 

 how distinctly different must be the mode of intrusion of the ultrabasic mass and associated 

 gabbros, "forming sills in dislocated mountains, which sometimes follows the planes of move- 

 ment and sometimes the plane of bedding," and the mechanics of intrusion of granite-batholiths. 



The distribution of rock-types within this narrow strip is significant. The serpentine 

 has been almost all derived from harzbergite, a minor amount from lherzolite; pyroxenite is 

 rare, but a small amount of pure enstatite rock occurs. In the northern end of the belt must 

 have been the locality where was found the aggregate of picotite which Judd ('95) described 

 as a vein of picotite rock. This the writer could not rediscover. Gabbros occur here and 

 there, generally confined within the sheet-like mass of ultrabasic rock, and but rarely pro- 

 truding therefrom to invade the adjacent sediments. They are more abundant proportionately 



