206 Pirsson — Microscopical Characters of Volcanic Tuffs. 



silica may be found until eventually they may pass into 

 extremely dense flinty rocks, resembling so closely dense 

 rhyolites, novaculites, jaspers, etc., that it becomes impossible 

 to distinguish their real nature in the field, more especially as 

 they may assume lively colors of green, red, yellow and brown. 

 Some striking instances of these rocks have been found in 

 North Carolina and Georgia among the ancient volcanics of 

 the Piedmont plateau. In thin section evident remains of the 

 vitroclastic texture may sometimes be observed, establishing in 

 connection with the shattered condition and nature of the 

 accompanying crystals, their tuff-like character ; but often 

 these are not present in any decisive fashion and in this case, 

 while chemical analysis may prove their igneous origin, in con- 

 trast with jaspilites and novaculites, it cannot be determined 

 whether one is dealing with tuffs or the lavas corresponding to 

 them. There is apt to be in these cases thorough devitrifica- 

 tion, as described later, which is another obscuring element. 

 Where the tuffs have been merely altered by weathering the 

 particles become turbid from the separation of kaolin, hydrated 

 iron ore, and carbonates, and lie in an impure cement of 

 minute granules of these substances which tend to obscure the 

 characteristic structure. An excellent example of an altered 

 tuff has been figured by Professor Bascom* from South Moun- 

 tain, in which the structure is well preserved. Some altered 

 tuffs, more especially those of the less siliceous and more 

 andesitic types, appear almost wholly like aggregated masses of 

 carbonates in the section ; one feels inclined to believe at times 

 that they must have been overlain by limestones whose sub- 

 stance has been leached down and deposited in them. Mingled 

 with these carbonates are earthy masses of iron ore. 



Probably from a somewhat different type of alteration the 

 fine tuffs may be changed into minute scales of sericitic mica 

 mingled with granules of quartz. Along with them is apt to 

 occur leaves and shreds of chlorite. It is often extremely dif- 

 ficult, or even impossible, to distinguish in the finer granules 

 whether one is dealing with sericite or kaolinite ; a difficulty 

 alluded to by Rosenbusch.f In basaltic tuffs, as in the so- 

 called palagonites, the alteration leads to the formation of 

 secondary silica, zeolites, chloritic minerals, carbonates, and 

 limonite. The cement between the ash particles is first at- 

 tacked and as the alteration proceeds with the particles 

 themselves, the original structure may be more and more 

 obscured until it is finally lost. Yet even when the vitro- 

 clastic texture has disappeared, the fragmental appearance 



* Ancient Volcanic Rocks of South Mountain, Penna., Bull. 136, U. S. G. S.. 

 PL XXV, 1896. 



f Mikrophysiog. Ergussgesteine, 4th Aufl., p. 870, 1908. 



