64 J. W. Judd — On Volcanos. 



without the appearance in the mass of any definite arrangements of 

 crystallites. 1 



Second Series. — Much more frequently, however, the passage from 

 the vitreous to the stony series takes place by the appearance in the 

 mass of scattered " sphasrulites," composed of radiating crystals of 

 felspar, entangling others of quartz, magnetite, and other minerals. 

 Occasionally these sphasrulites are found scattered in a promiscuous 

 manner through the vitreous matrix ; but, far oftener, they assume 

 very striking and definite arrangements ; these are clearly seen to 

 be the result of the conditions of pressure, tension, and slow- 

 dragging movements to which the slowly consolidating mass was 

 subjected. Sometimes the alternate laminee of vitreous or colloid 

 and stony or crystallized materials have assumed a parallel arrange- 

 ment, and the rock is almost as perfectly cleaved as a piece of slate ; 

 at others they assume all the beautiful wrinklings and corrugations 

 so characteristic of metarnorphic foliated schists. The light which 

 these remarkable products throw upon the mode of formation of 

 many of the older rocks will be illustrated on a future occasion. 



Third Series. — At times the obsidian base of the rock is porphy- 

 ritic, that is to say, it has crystals, often large and well formed, most 

 commonly of brilliant sanidine, but not unfrequently of quartz, 

 hornblende, or black-mica, floating through its mass. It then as- 

 sumes the characters of an " obsidian-porphyry " (porphyritic 

 obsidian). No one can study this rock, as exhibited in Lipari, 

 without being convinced that the crystals which it contains were 

 ejected, ready-formed, with the lava as it issued from the volcanic 

 vent. Not only is there no trace of ciystals in various stages of form- 

 ation, as in the case of the sphasrulites, etc., but sometimes pumiceous 

 masses, evidently blown out of a volcanic vent, may be found en- 

 tangling just such perfect crystals. We shall not at present enter on 

 the discussion of those interesting problems which the phenomena of 

 these perfect crystals of minerals of such different degrees of fusi- 

 bility, floating in the same liquefied highly siliceous magma, must 

 suggest to every geologist. We shall only notice, in this place, that 

 the combinations of these ejected crystals with those gradually 

 developed in the mass by the growth of crystallites, the whole 

 modified by the peculiar mechanical conditions to which the masses 

 have been subjected, result in the formation of rocks of wonderful 

 diversity, exquisite beauty, and remarkable suggestiveness to the 

 petrologist. 



Fourth Series. — Fresh complexities of rock structure are originated 

 and new varieties of lava produced, when, in either of the kinds 

 already noticed, disengagement of volatile materials in the midst of 

 the mass began to take place. The vesicular cavities thus originated 

 were variously modified by the strains and movements to which the 

 plastic mass was subjected. The most stony and highly crystalline, 

 as well as the most vitreous varieties of these lavas, are thus affected 



1 The exceedingly beautiful and clear obsidian of Lipari, like that of Mexico, has 

 been employed by the ancient inhabitants of the island for cutting instruments and 

 weapons. 



