i6 HENRY S. WASHINGTON 



divergently arranged. Between these feldspar laths is a granular 

 aggregate of small, colorless augite anhedra, which are generally 

 equant rather than prismoidal, and many opaque grains of magnetite, 

 small grains of olivine and apatite prisms being rarely seen in the 

 ground-mass. While some of the specimens seem to be holocrystal- 

 Hne, others are distinctly vitreous, and in these the femic constituents 

 are not well crystallized, but are represented by dusty, dark specks, 

 which render the ground-mass almost opaque. This is especially 

 well seen in the rock which forms the dike at Monte Pozzolana. In 

 this type nephelite seems to be rare or wholly wanting. 



Mode. — The small size of the ground-mass constituents, especially 

 of the augite and magnetite anhedra, and the great extent of over- 

 lapping, do not permit a satisfactory estimate of the mode by Rosi- 

 wal's method. But by making certain readjustments of the norm 

 as calculated from the chemical analysis, the mode of the upper north 

 flow of Monte Ponente was estimated to be approximately as follows : 



Labradorite (x^biArii) 57 



Augite . 26 



Olivine 6 



Magnetite (titaniferous) 10 



Apatite i 



100 

 The abundance of labradorite, and the relatively much greater 

 amount of augite than of olivine, are the distinguishing features of 

 this mode. 



Chemical composition. — Two analyses of this type were made by 

 me, one being of the uppermost, and the other of the lowest accessible, 

 north flow of Monte Ponente. Parts of this latter flow showed a 

 white mineral (gypsum) in the vesicles, but a specimen free from this 

 was chosen for analysis. The gypsum itself will be described later. 

 In this, as well as in the other analyses made of the basalts, especial 

 attention was paid to the determination of titanium and nickel, the 

 presence of which is so often disregarded, as these constituents have 

 been found to be of considerable importance in considering the corre- 

 lations of the Linosa rocks. Analysis No. II has been previously 

 published,^ but in a less complete form. 



I H. S. Washington, Quarterly Journal of the Geological Society, Vol. LXIII, 

 1907. P- 75- 



