April 14, 1905.] 



SCIENCE, 



577 



appeared within the old crater (Etang Sec) 

 since the eruption of May 8, 1902, and which 

 was considered by some of the American ob- 

 servers as merely a secondary cinder cone, or 

 an accumulation of fragrtiental ejected ma- 

 terial about the actual vent. Lacroix denies 

 that it is of fragmental nature and states that 

 it is, in fact, a homogeneous mass of viscous 

 lava surrounded by an envelope of the same 

 substance cooled and consolidated. The ex- 

 planation of this phenomenon and of the re- 

 markable spine of solid rock which has from 

 time to time risen above the dome itself is 

 essentially as follows: 



The viscous magma on reaching the surface 

 through the throat of the volcano and forming 

 a protuberant mass is quickly surrounded by 

 a solid shell or envelope which protects the still 

 pasty interior from a too rapid cooling. This 

 envelope becomes fissured, under the influence 

 of progressive solidification, and the increase 

 in volume of the mass itself, and through the 

 clefts thus formed fresh molten material is 

 exuded. In this way a homogeneous rocky 

 mass increases in height and volume, bristling 

 with jagged points, glowing like a ' charcoal- 

 burner's fire ' at night, and bounded by abrupt 

 walls which rise from the debris accumulating 

 at its base from incessant superficial crum- 

 bling. Projected materials resulting from 

 violent eruptions play but a small part in the 

 constitution of such a dome. A dome so con- 

 structed is not characterized by any perma- 

 nent vent or crater, but violent eruptions de- 

 termine such openings, which are of a tem- 

 porary nature and rapidly closed. When the 

 envelope has become sufficiently resistant to 

 permit no longer a growth in all directions, the 

 action of the pressure is localized at certain 

 points and results in the extrusion of solid 

 rocky masses issuing as from a mold and pro- 

 ducing needles or spines which may attain an 

 elevation of several hundred meters. In the 

 course of a single eruption the point at which 

 the pressure concentrates itself may be dis- 

 placed, and successive spines may result of 

 diverse forms, dimensions and positions, and 

 often of an ephemeral existence, for continual 

 crumbling tends to modify or totally destroy 

 them. This process, which has been unrecog- 



nized up to the present, and which Lacroix is 

 the first to describe, explains, in his opinion, 

 the origin of the many peculiarities of struc- 

 ture of ' domes ' in volcanic regions where 

 activity no longer is manifest, and which have 

 often been assumed to be the cores of dissected 

 volcanoes. 



Four types of rock were recognized as con- 

 stituting the dome and spine, the specimens 

 being collected from the talus extending down 

 the Riviere Blanche from the base of the 

 dome, and from the edge of the old crater 

 after violent eruptions. The four types are 

 differentiated by the character of their ground- 

 masses, the phenocrysts being the same; they 

 are essentially hypersthene andesites. Rocks 

 of the first two tj^ies are glassy, the first being 

 obsidian, the ground-mass of the second con- 

 taining a few microlites of an acid plagioclase 

 in a glassy base. The third type is a porous 

 or vesicular andesite containing greater or less 

 amounts of plagioclase microlites in the 

 ground-mass; the fourth type differs but little 

 from the third except that the groundmass 

 contains quartz in small crystals frequently 

 abundant enough to make the rock micro- 

 granular. Of these four types I. and II. are 

 believed to have been derived from the shell or 

 envelope of the dome, type I. probably repre- 

 senting the original crust, and type H. the 

 material filling the cracks and fissures in the 

 outer shell. Rocks of these types are abun- 

 dant in the ejectamenta of violent eruptions. 

 Specimens of types III. and IV. w^ere obtained 

 from the quiet crumbling or breaking down 

 of the spine. The quartz-rich rocks are as- 

 sumed to have come from the interior of the 

 spine and are the result of a much slower 

 cooling of the magma, while rocks of type III. 

 represent a somewhat more rapid cooling. In 

 commenting upon these quartzose andesites, 

 Lacroix says ; " * * * The actual production 

 of quartz-bearing rocks, more or less com- 

 pletely microgranular, which I consider one 

 of the most important observations that I had 

 occasion to make, shows that the conditions 

 necessary for the formation of quartz in a 

 volcanic rock may be realized at slight depth 

 under the solidified outer shell of a dome of 

 acid rock in process of evolution " (page 157). 



