28 HENRY S. WASHINGTON 



THE TUFFS 



While volcanic tuffs, as a rule, offer few features of interest, yet 

 as those of Linosa are so abundant and play such a conspicuous role 

 in the formation of the cones of the earlier period, they may be 

 described briefly. 



These tuffs vary in color from dark, rather greenish, gray to a Hght 

 brownish yellow. In some cases only one of these is present, as yel- 

 low tuffs at Monte Pozzolana and Monte Biancarella, or dark gray 

 at the main ridge of Monte Bandiera; while elsewhere both varieties 

 are met with, as at the two craters east of Monte Bandiera and at 

 Monte Levante. As will be seen later, the differences between the 

 two are probably to be ascribed only to differences in the progress of 

 alteration. 



The dark-gray tuffs are very fine grained and friable, readily 

 rubbing down to an impalpable powder, at least in great part. Aside 

 from the large blocks of lava which are embedded in them, they con- 

 tain very small fragments of black basalt, seldom larger than a pea 

 and running down to very small angular grains. A bedded structure 

 is generally clearly manifest, and is frequently emphasized by thin 

 layers more rich in the small fragments of basalt. Unfortunately, 

 no sections were made of these, but the powder, when viewed under 

 the microscope, shows fragments of colorless augite as the only recog- 

 nizable mineral, the greater part of the mass being a dust of almost 

 opaque, but apparently isotropic, minute grains, probably of glass. 



The yellow tuffs^ show, Hkewise, small pieces of black basalt, often 

 vesicular, which stand out against the light-yellow background more 

 prominently than do those in the gray tuff. The yellow portion is 

 rather less friable than the preceding, and is somewhat porous, the 

 constituent grains being agglomerated into very small, rounded 

 aggregates, a millimeter or less in diameter, between which there are 

 numerous crevices and pores. The powder of this, under the micro- 

 scope, also reveals small fragments of augite, the light-yellow dust in 

 which they lie being semi-transparent and apparently isotropic. 



The chemical composition of these tuffs is shown in the two analyses 

 given below, both analyses being made of average material which 



I These are the trachytic rocks of Calcara. 



