RECENT VOLCANIC ROCKS 321 



In mineralogic composition these recent volcanic products consist very 

 largely of hypersthene-andesite. The last eruption from Masaya was a 

 basaltic lava, and a comparatively recent lava flow from Ometepe is also 

 a basalt. With these two exceptions the recent activity, so far as ob- 

 served, has given rise only to andesitic lavas and tuffs. Th« cone of 

 Ometepe consists largely of lapili, with occasional interlaminated lava 

 flows. The lapili consist in about equal parts of black or gray pumice 

 and of black glassy rock, which has been thoroughly shattered and 

 ground up by explosive eruptions. The tuffs from this volcano, which 

 have been carried to a considerable distance from the center of eruption 

 chiefly by wind, are composed more largely of tuffaceous material. 



The materials erupted from the other volcanic centers forming the 

 various peaks of the Nicaraguan range appear to be similar in compo- 

 sition to those found in Ometepe. The Jinotepe plateau is composed 

 largely, if not altogether, of volcanic tuffs, which probably reached their 

 present position in the form of a more or less fluid mud. This mud be- 

 comes solidified, but never sufficiently so to form hard rock. It is quar- 

 ried in many places and used as a building stone. It can be readily cut 

 out with a pick, but becomes somewhat harder on exposure to the air. 

 The rock fragments which constituted this tuff vary widely in size, from 

 large boulders several feet in diameter to the finest dust. They are all 

 angular, and in this respect they differ from the volcanic conglomerates 

 associated with the Tertiary rocks of the San Juan valley. A further 

 difference is the almost complete absence of stratification and sorting of 

 the rock constituents. This tuff appears to have been sufficiently fluid 

 to flow on rather low slopes, and the present southward and westward 

 slopes of the Jinotepe plateau are probably the original constructional 

 slopes. 



In the vicinity of Managua planes separating successive mud flows 

 intersect the rock and are utilized in quarrying. In these quarries human 

 tracks have been found in the rock, where they were made while it was 

 still in the form of mud. They prove the recency of the tuffs and indi- 

 cate something as to its physical condition when first deposited. At the 

 margin of this plateau the tuff is found filling the valleys in the older 

 formations and smoothing out the former irregularities of the topography. 

 In many cases the present streams have in part reexcavated the old 

 valleys, though not to their original width. 



The vertical cliffs surrounding the caldera lakes, Apoya and Masaj^a, 

 display the underlying structure of this plateau near the centers from 

 which its material was derived. These cliffs are composed of alternat- 

 ing layers of tuff and solid lava flows. It is impossible to say how far 

 from the centers of eruption these lava flows extend, but the distance is 



