110 J. D. Dana — Metamorphism from Volcanic Conditions. 



lar and tabular crystals, besides black octahedrons of magnetite. 

 The microscopic investigation of Mr. Dana proves that they 

 actually are like the lavas in constitution, and that the crystals 

 are of pyroxene and feldspar as in the ejected blocks. 



The origin of the stalactites of the tunnels and their crystal- 

 lizations is due, as I state in my Expedition Report (p. 201), to 

 "the action of steam on the roof of the cavern." In the case 

 of the tunnels the flowing lavas left behind a chamber tilled 

 with superheated steam, and nnder its action the solution and 

 recrystallization went forward. 



This reproduction of the basalt and the making of the crys- 

 tals in geodes, or as linings of fissures, are examples of metamor- 

 phic work. It is metamorphism of the crystallinic kind", the 

 same which takes place when a feldspathic sandstone is converted 

 into granite or granulyte, or when calcyte is changed into mar- 

 ble ; and it is therefore one of the common kinds of metamor- 

 phism. 



•i. The ejected blocks about Kilauea instruct us on another 

 point of much geological importance. They show that the 

 throat of a volcano is necessarily a region of metamorphic ac- 

 tion. It is a region of continued heat ; and heat always works 

 change when moisture is present. IJnder such conditions, 

 therefore, an Archaean limestone or other Archsean rock con- 

 taining chondrodite, spinel, vesuvianite, scapolite, anorthite, 

 nephelite, biotite, might lead to the production of recrystallized 

 chondrodite (humite), spinel, vesuvianite, scapolite (meionite), 

 anorthite, nephelite, biotite (or meroxene) as metamorphic re- 

 sults ; and in just the situation where an explosive eruption 

 might detach masses and bring them up to the light. It is 

 noteworthy that the above minerals of the ejected blocks about 

 Somma, which have long been regarded as throat minerals of 

 Vesuvius, crystallized by the volcanic heat as held by Scacchi, 

 are kinds that are characteristic of Archsean rocks and espe- 

 cially of an Archsean limestone, rocks which may underlie the 

 later limestones and other strata. There is little assumption 

 therefore in saying that some of these crystallizations illustrate 

 specifically crystallinic metamorphism, though others may be 

 of the me'tachemic kind, that is, products of chemical change. 



From the side of a fissure near the bottom of the emptied 

 basin of the lava-lake called the " Old Beggar," was taken, at 

 my visit in August, 1887, a specimen as large as the hand, cov- 

 ered with minute white tabular crystals, with some transparent 

 crystals of acicular form. The mineral turned out to be gyp- 

 sum, common as an incrustation in Kilauea caves. 



* •• On terms applied to metamorphism," this Journal, III, xxxii, 70, 1886. 



