Physiography of Bulla Area. 339 



a granodiorite mass had been converted into kaolin at various 

 places by pneumatolysis long before the lava floods. At these places 

 the weathering would be >nuch more rapid than where the rock 

 was unattacked. Depressions would be made, and, later, occupied 

 by the lava, wliilqj the fresh and less denuded rock would stand up 

 as a monadnock above the molten basalt. 



(d^) Fresh Granodiorite -under Basalt. — In two places fresh,, 

 hard granodiorite was seen directly beneath basalt at a low level, 

 while the granodiorite to the side of it had been kaolinized. At 

 another place granodiorite was seen under basalt that had flowed 

 into an old valley. There was no sign of kaolinization. 



(e) Shape of the Outcrops. — In this area the shape of the outcrops 

 is not very definite, but where it is shown it agrees with type (a), 

 (Fig. 1, Plate XXXIIl.) If the kaolin were of meteoric origin, it 

 would be of type (b) (Fig. 2, Plate XXXIIL). 



(f) Relation of Kaolin to Depth. — In every case the rock near 

 the bottom of an outcrop is as much altered as that higher up the 

 face. If the decomposition were due to surface w'ater and carbon. 

 dioxide, then decomposition should decrease as the depth increased, 

 for the solid rock deep under the surface stream would suffer very 

 little from downward drainage. 



(g) No Evidence of Stream Beds above the Kaolin. — If the 

 kaolin were formed by Avater in old valleys draining through grano- 

 diorite, some trace of strea^pi material above the kaolin would very 

 likely be found. No trace of river gravels, silt or conglomerate was 

 found above any of the kaolin masses. 



(h) Kaolin Found at Many Levels.— Kaolin can be found right 

 from the level of the stream to 200 ft. above it. One would not 

 expect such differences in level in separated outcrops if old valley 

 floors determined the point of attack on the granodiorite by the 

 stream water with carbon dioxide in solution. 



(i) No Evidence of Kaolin Being Formed in Present Stream. — 

 The granodiorite along Deep Creek and on the hill sides is not 

 kaolinized. If kaolinization be due to subaerialj agencies, why has 

 not the granodiorite in this stream bed been kaolinized? 



(j) Accompanying Minerals.— Kaolinization by pneumatolysis is 

 generally accompanied by the production of tourmaline, fluor, cassi- 

 terite or topaz, which cannot be produced by subaerial agents. A 

 vein of tourmaline in Q^ and another in Q^ (Plate XXXII.) give 

 valuable positive evidence that magmatic vapours have been pre- 

 sent to some extent at least. 



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