2 54 HENRY S. WASHINGTON 



black masses, which often attain diameters of five centimeters. 

 These are coarsely crystalline and not very compact, with 

 miarolitic and vesicular cavities. Under the microscope the 

 only phenocrysts visible are of pyroxene. These are usually well 

 formed, but occasionally quite fragmentary. The interior con- 

 sists of a colorless diopside, which is uniformly surrounded by a 

 rather narrow border of yellowish green non-pleochroic augite; 

 The substance of this border corresponds exactly with that of 

 the small groundmass augites, so that it is due to the same late 

 period of growth to which they belong. The large augites are 

 hence a good instance of the growth and enlargement of crystals 

 brought up from below by the late accretion of isomorphous sub- 

 stance of somewhat different composition. Apart from the not 

 very abundant small augites and magnetites, the groundmass is 

 made up of feldspar and some residual glass. The feldspar forms 

 laths with somewhat ragged edges, and they, with the augites, 

 show well-marked flow structure. The greater part of these 

 laths are of orthoclase, or at least an alkali feldspar ; only very 

 few showing twinning striations and extinctions which would 

 refer them to plagioclase. No suitable sections of these latter 

 were found by which their approximate composition could be 

 determined, though I am inclined to think that they belong to 

 the middle of the plagioclase series rather than to the basic 

 end. The powdered rock, on treatment with acids, furnishes 

 abundant gelatinous silica, which probably comes from the glass 

 base. The dark spots referred to above are seen under the 

 microscope to be holocrystalline, and composed of a few large 

 diopsides with augite borders and very many smaller prismatic 

 augites lying in a cement of orthoclase or alkali feldspar. The 

 line of junction with the surrounding rock is quite sharp. They 

 are true segregations and are to be classed with the enclaves 

 homcBoghies of Lacroix.^ 



Bucca also mentions briefly the rock of Mte. Ofelio, near 

 Sessa, on the southwest flank of the volcano, as a true trachyte. 

 It seems to be much decomposed. 



' Lacroix, Enclaves des Roches, Macon, 1893, 8. 



