332 RECORD OF SCIENCE FOR 1887 AND 1888. 



lavas and pitch-stones remain unchanged. In the field this distinction 

 is shown by the piimiceous condition oY the obsidian and massive form 

 of the stony lava and pitch-stone. From these and tlie facts that tlie 

 porphyritic constituents of the obsidian occur in " glomero-porphyritic" 

 forms he argues tliat this obsidian is but a refused portion of the older 

 Krakatoa lavas, and, after a discussion of Dr. Guthrie's experiments 

 on the influence of water in lowering the fusion points of mineral sub- 

 stances, proceeds to argue that this refusion and perhaps the volcanic 

 eruption of Krakatoa itself was brought about through the admission 

 of water in the mass of mixed silicates buried at depths below the sur- 

 face. The mass of anhydrous rock might be in a solid state at a compar- 

 atively high temperature, while the same rock rendered hydrous through 

 the gradual percolation of water would ultimately fuse and give rise to 

 all the phenomena of the eruption. As expressed by Guthrie, "the 

 phenomena of fusions is nothing more than an extreme case of lique- 

 fiiction by fusion," it being impossible to tell where liquefaction leaves 

 off and fusion begins. 



CONTACT METAMORPHISM. 



'Several fine illustrations of contact metamorphism have been described 

 during the two years covered by this report and mention may here be 

 made of a few of the more interesting and important. 



Dr. G. H. Williams* conchules an admirable series of papers on the* 

 rocks of the "Cortland series,"nearPeekskill,]S'. Y., with a description of 

 their contact metamorphisms or phenomena produced by eruptive rocks 

 on the adjacent schists and limestones. The erui)tive, or dike rocks in 

 this case are norites, gabbros, peridotites, mica or mica-hornblende dio- 

 rites, pyroxenites, and hornblendite. The schists are highly crystalline* 

 schistose rocks consisting of quartz and feldspar, with both muscovite 

 and biotite together with tourmaline, magnetite, and zircon. Approach- 

 ing the line of contact the schists become more and more puckered and 

 contorted and filled with lens-shaped " eyes "of quartz containing garnets 

 and other contact minerals. In the schists themselves are developed 

 staurolite, sillimanite, cyaiiite, and garnet, the amount of metamorjdiism 

 being directly proportional to the nearness tothe line of contact. At con- 

 tact the schistose structure is almost completely obliterated and the 

 rock becomes luird and massive, appears more or less fused with the nu'ca 

 dioiite, is highly garnetiferous, and consists of a great variety of miuer- 

 jils, including staurolite, sillimanite, pyroxene, green hornblende, dial- 

 liige, sc:ii)olite, and sjdiene. Briefly expressed, the progressive change, 

 approaching the line of contact consist in a gradual decrease in the 

 amount of silica and the alkalies with a corresponding increase in iron 

 and alumiiiai, this being accompanied by a disappearance of the (piartz 

 and muscovite and the development of biotite, sillimanite, staurolite, 

 cyauite, and garnet. In fragments of the schist taken up by the erup- 



Aui. Jonr. of Science, October, 188)^ p. 254. 



