HEAT. 289 



iibers, as if on crystallizing in these dendritic forms the latter mineral had 

 drawn the feldspar into parallel position with it." Fig. 253 represents the 

 facts well, as the feldspar crystals are larger than usual. Fig. 252 shows 

 one large ordinary crystal of pyroxene below the tuft. Figs. 254—256 repre- 

 sent some of the microlites in the basalt-glass of the region. (E. S. D., 1888.) 



In the consolidation of igneous rocks a more decided concretionary structure some- 

 times results. This is especially true of glassy or semi-glassy kinds, which often contain 

 spherulites, having more or less distinctly a radiately fibrous structure. Spherulites appear 

 to differ little from the radiate concretionary forms common in manufactured glass which 

 has been artificially devitrified. (Rutley, 1890.) Some spherulites are in part separated 

 peripherally from the inclosing glass, as if formed within " lithophyses " or vesicles. 

 (Iddings, 1888.) See page .337, beyond^ under Metamorphism. A concretionary form in 

 dioryte is represented on page 97. 



(6) Volcanic bombs. — Volcanic bombs are roundish or ovoid masses of 

 lava, concentric in structure. They sometimes have a center of chrysolite, 

 or of the more scoriaceous lava. They occur on Hawaii in connection with 

 the aa, and are of various sizes, from one inch to ten feet or more in diameter. 

 They are produced on Hawaii by the rolling movement of the front of the 

 stream due to friction at bottom. It is possible that the same kind of move- 

 ment in the ordinary lava-stream may produce them ; but on Hawaii they are 

 found only in aa lava-fields ; one is shown in Fig. 249 at a. Johnston-Lavis 

 gives essentially the same general explanation of the origin of some bombs 

 observed by him about Vesuvius. The bombs of the Eifel region, in many 

 of which chrysolite makes the center, have been supposed to be projected 

 bombs ; but in view of the above facts this may be questioned. Projected 

 blocks of ordinary lava are not bombs, but merely projected blocks. 



(c) The opening of subordinate or lateral volcanic cones. — Cones of erup- 

 tion often form over fissures during the progress of an eruption from the 

 fissure. Each such cone, when it is in progress, has its own lava-conduit, as 

 a branch from the general lava-conduit of the mountain. But it is relatively 

 small, and its liquid lavas consequently may soon become chilled by the 

 cold rocks about it; and hence such lateral or subordinate volcanoes have 

 usually a brief existence. They, however, often work hard during their 

 short life, and even in two or three weeks may make a cone many hundred 

 feet in height. 



Such cones occur about the sources of great eruptions ; but they are most 

 common near the seashore, where subterranean fresh waters most abound 

 for the supply of moisture, and where the sea is at hand as another 

 source. They may be either cinder-cones or tufa-cones, but are most 

 likely to be the latter if near the seashore. The volcanic origin of such 

 €ones can be proved by the 2^^'>^^<^^'>^t'^'i'(^ arrangement of the materials con- 

 stituting them. The sea, with its broad waves and the aiding winds, can 

 make heaps or ridges out of the sands existing or produced on its borders, 

 but it cannot arrange the layers of sand or earth pericentrically into a 

 Dana's manxjal — 19 



