722 DYNAMICAL GEOLOGY. 



steam in California, north of San Francisco, in Geyser Canon, a 

 branch from Pluton Canon. The waters have a temperature, accord- 

 ing to Whitney, of 206° to 207°; and deposits of sulphur are formed 

 from them. Near Clear Lake, there is a " Borax lake," holding borax 

 in solution, and having a deposit of it, over its bottom ; and, as Whit- 

 ney observes, it is evidence of the action of hot waters in former 

 times. Boracic acid is held in solution in the hot waters of the 

 Tuscan lagoons. Other beds of borates occur in southern Oregon, 

 near the sea, and in Nevada and Arizona. 



3. Structure of Rocks produced by Cooling The sketch on 



page 717 illustrates the columnar structure common in many kinds of 

 igneous rocks. It results from contraction on cooling and concretion- 

 ary solidification (pp. 86, 87). By the same means, a rock often be- 

 comes irregularly cracked, or regularly jointed. Moreover, an argil- 

 laceous rock, or even a sandstone, in the vicinity of a dike of igneous 

 rock, is often columnar, in consequence of cooling after having been 

 heated up by the rock of the dike at the time of its ejection. A good 

 exhibition of columnar sandstone occurs near a trap dike at New Ha- 

 ven, Conn. The columnar structure is always at right angles to the 

 cooling surfaces, as stated on page 421. 



The outer part of a dike, where it adjoined the enclosing rock, is 

 often much cracked, and also of very much finer texture than the in- 

 terior, because it was most rapidly cooled ; and obsidian, or volcanic 

 glass, is an extreme effect of rapid cooling (p. 702). 



4. Sources of Igneous Eruptions. 



1 . The Earth's interior Fire-seas. — The existence of a liquid layer, 

 or of great fire-seas, beneath the crust, being a fact, volcanoes are nat- 

 urally regarded as outlets to the surface for the liquid rock. The great 

 extent of the lines of volcanoes about the globe (p. 703), and also of 

 the regions of igneous eruption in all ages, to the present time, shows 

 that the interior fire-seas have been and still are large. Over a range 

 of country 1,000 miles long, from Nova Scotia to South Carolina, the 

 eruptions at the close of the Triassico-Jurassic era were all dolerytic 

 (p. 78), proving the oneness of the sea of fire at that time beneath 

 the Atlantic Border. Far greater eruptions took place in the Tertiary 

 and early Quaternary eras, over the Pacific slope of North America. 



But, supposing a great fire-sea, or a general liquid layer to be the primary source 

 of volcanic action, it does not follow that a connection with the same is now retained. 

 After the eruption from a fracture had continued for a while, the connection may have 

 becoVne cut off by cooling, so as afterward to extend only to a subordinate reservoir of 

 liquid rock. When several volcanoes have been opened on a single profound fracture, 

 they may afterward have become wholly disconnected from one another, and also from 

 the earth's interior. Kilauea, on the flanks of Mount Loa, is one of the largest volcanic 



