138 THE CRYSTAL PALLS IRON-BE ARIXG DISTRICT. 



their progress tliroi;a-li the air. (PI. XIII.) They are for the most part 

 not scoriaceous, though rather commonly amygdaloidal. The macroscopi- 

 cally dense fragments seem to predominate, though the amygdaloidal ones 

 do occur in some specimens in nearly equal quantit5^ 



The fragments of the tuffs are derived from the various kinds of basalt 

 already descriljed as forming the lava flows. 



Among the fragments some of the most typical of these rocks have 

 been found, and remarkable as it may seem, some of the thin sections from 

 them show the least-altered basalts. 



In addition to the kinds mentioned under the basalts there are a number 

 which differ slightly from them, and apparently represent more glassy modi- 

 fications of the basalt magma. In one of these the amygdules are more 

 sharply outlined by the accumulation of iron oxide around the edges of the 

 amygdule than is the case in the crystalline flow rocks. An especially 

 well-preserved fragment shows perfectly fresh plagioclase microlites exhib- 

 iting well-developed fluidal structure lying in a dark-brown apparently 

 isotropic glassy base. Where the section is thin, globulitic devitrification 

 products can be seen, and there also the base no longer appears isotropic, 

 but very feebly double refracting. There is very frequently found among 

 these tuffs amygdaloidal fragments which appear to have been derived from 

 what was originally a completely glassy rock, no indication of the presence 

 of any original crystals having been preserved. The background of these 

 fragments consists of a fine felt of a green chloritic mineral, dotted with 

 innumerable grains of epidote, in which one may distinctly discern concen- 

 tric circles and arcs of circles outlined by aggregates of epidote grains. 

 These circles probably represent perlitic partings. (Fig. A, PI. XXXII.) 

 The dark-brown fragments mentioned as occurring with the prevailing green 

 ones are verT" dense, appear to be very rich in iron, and may possibly 

 represent a very basic devitrified glass. Should accumulations composed 

 essentiall}" of such glassy fragments be found, they could properly be 

 called "palagonite tuffs." 



In addition to the rock fragments, a few rare ones of large plagioclase 

 crystals were found, and also in one case a fragment of a violet-brown 

 augite, the only specimen of fresh pyroxene thus far found in any of the 

 volcanics. 



The tuffs show in places fairly well-developed banding, caused by the 



