832 RECORD OF SCIENCE FOR 1887 AND 1888. 



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

 is shown by the puniiceous condition of the obsidian and massive form 

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

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

 forms he argues that 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 niineral 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 migh t 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- 

 faction 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* concludes an admirable series of papers on the 

 rocks of the ' ' Cortland series, " near Peekskill, N. Y., with a description of 

 their contact metamorphisma or phenomena produced by eruptive rocks 

 on the adjacent schists and limestones. The eruptive, 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 blotite together with tourmaline, magnetite, and zircon. A])proach- 

 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 

 stanrolite, sillimanite, cyanite, and garnet, the amountof metamorphism 

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

 tact the schistose structure is almost completely obliterated and the 

 rock becomes hard and massive, appears more or less fused with the mica 

 diorite, is highly garnetiferous, and consists of a great variety of miner- 

 als, including stanrolite, sillimanite, pyroxene, green hornblende, dial- 

 lage, scapolite, and sphene. J>riefly expressed, the progressive change, 

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

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

 and alumina, this being accompanied by a disai)j)earan('e of the (piartz 

 and muscovite an<l the development of biotit<', sillimanite, stanrolite, 

 cyanite, and garnet. In fragments of the s(;hist taken up by the erup- 



Aiii. Jour, of Science, October, 18S6, p. *54. 



