140 a. H. WILLIAMS — SILICIFIED GLASS-BRECCIA. 



indebted to the skill of Mr. Charles R. Keyes, Fellow in Geology at the 

 Johns Hopkins University. 



The fragments, even down to those of the smallest dimensions, have the 

 angular form characteristic of glass sherds produced by explosive eruptions. 

 The larger fragment in the lower part of the figure is finely vesicular, while 

 the one above is more coarsely so. The flow structure is as perfectly marked 

 by sinuous lines of globulites and microlites, which terminate abruptly against 

 the broken edge of the glass particle, as in the most recent vitrophyre. 

 Minute spots of opaque pyrrhotite are scattered through the section. The 

 groundmass is of a dark color, owing to the massing in it of minute black" 

 globulites, to whose nature the highest magnifying power gives no clue. 



Unfortunately, no analysis of this interesting rock has as yet been made. 

 Between crossed IS^icols it is seen to be made up largely of chalcedonic 

 quartz, which has changed the easily destructible glass into a sort of jasper. 

 Chlorite is also abundant, frequentl}^ arranged as a border of radiating 

 scales around the edges of the fragments, so as to coat them green in the hand 

 specimen. The larger grains are always a fine mosaic of interlocking quartz, 

 but some of the smaller ones are composed of a unit individual of clear vitre- 

 ous quartz. The only other minerals which could be identified in the sec- 

 tion are calcite and a few grains of a glassy, striated feldspar. The presence 

 of this latter mineral is very noteworthy, as we should expect it to have dis- 

 appeared during the vicissitudes through which this rock has passed. 



After a careful study of this rock I find it possible only to interpret it 

 as a remarkable instance of a very ancient volcanic glass-breccia, preserved 

 through the lucky accident of silicification. Nor did this process go on, as 

 is usual, through dentrification and loss of structure, but rather like the 

 gradual replacement of many silicified woods, whose every minute detail of 

 structure is preserved. The rarity of such rocks in the earth's oldest forma- 

 tions is readily intelligible, but for this ver}' reason the exceptional preser- 

 vation of a rock like this is all the more welcome proof that explosive 

 volcanic activity took place at the surface, then as now, and on a scale, if 

 possible, even greater than that w'ith which we are familiar. 



