274 DYNAMICAL GEOLOGY. 



solutions, obtained crystals of nephelite and orthoclase ; and with the addition of silica, 

 obtained leucite. 



As early as 1804, Gregory Watt published in the Philosophical Transactions "Obser- 

 vations on Basalt," in which he gave a detailed account of the melting of 700 pounds of 

 basalt from Rowley-Rag (G = 2-743) to glass, and of its becoming, on slow cooling, a 

 gray, crystalline-granular mass (with G = 2-934-2-949) consisting of spherical concretions, 

 many 2 inches in diameter and having a somewhat radiated structure (which was mostly 

 lost with the slowest cooling) ; and of the adjoining concretions being often rendered 

 hexagonally prismatic from contact, whence he inferred the concretionary origin of 

 basaltic columns. 



2. Conditions Determining the Forms of Cones. 



1. Dependence on fusibility of the lavas. — Cones of lavas of the basalt 

 class are of gentle slope, and great breadth, owing to the easy flow of the 

 rock. The lavas are glassy only at surface, or when in scoriaceous forms. 



The craters also derive their characters from the liquidity. They are 

 broad, with the walls often vertical, meriting the name they have of pit- 

 craters, as is well seen in figures 229-231, on page 269. 



But the great cones of western North and South America are mostly 

 examples of the andesyte or trachyte class. The slope seldom exceeds 35°, 

 except where caused by breaks. The steepness, however, may be in part 

 owing to intercalated beds of cinders or tufa. Mount Shasta, represented 

 in Fig. 226, is one of them, — its slopes 28°-32° (Whitney). Chimborazo, 

 20,498 feet high, has angles of about 25° in a view looking northeast ; Coto- 

 paxi, 19,613 feet high, in a westward view, angles of 21\° to 30^-°, rising near 

 the summit to 37° (Whymper); and Arequipa, angles of 271° to 32° 50'. 



Trachytes, and other lavas of the third class, take part in cones of the 

 second kind. But as the temperature of free fusion is above 3100° F., the 

 heat required for complete liquidity is generally wanting, so that at the 

 time of ejection they commonly are already in a pasty state, or that of 

 incipient solidification. The streams are thick, compared with the basaltic. 

 Sometimes the lava swells up into steep and lofty craterless domes, instead 

 of flowing away in streams. The high domes of Auvergne, France, are 

 examples. But when a trachytic lava has the heat of complete fusion, it 

 may flow and make great streams. 



The following sketch represents "Gothic Mountain," in Colorado, in 

 which a mountain mass of trachyte rests on a base of Cretaceous rocks, 

 much eroded over its surface. (Hayden Rep., 1873.) In the nearly hori- 

 zontally stratified base there is an independent dike of the trachyte, which 

 was probably produced contemporaneously with the outflow that made the 

 mountain. The mountain is nearly 2000 feet in height above the Cretaceous 

 base, and 12,465 feet high above the sea level. The rock is without bedding 

 or any evidence of separate lava flows. 



Melted beeswax poured out on a flat surface, while heated above the 

 fusing point, would flow off at a very small angle ; but if its temperature 

 were below that of fusion, it would be pasty, and the angle of flow would 



