k 



PEALE.J RAPIDITY OF DEPOSITION FEOM THEEMAL SPRINGS. 399 



take a glance at the siliceous sinters we find that usually it is not so 

 great in quantity. The deposits of the Great Geyser, in Iceland, accord- 

 ing to Descloizeaux, in 1846, had a thickness of 13 to 16 feet, and the 

 more ancient deposits had a thickness of over 80 feet.* 



In New Zealand, Waikite, at Whakarewarewa, has a siliceous cone 100 

 feet in diameter and 15 feet high. Pohutu, at the same place, has 

 deposits 20 feet in thickness. In the Azores a thickness of 30 feet of 

 siliceous sinter is found. 



In the Upper Geyser Basin of Fire Hole Eiver, the Castle Geyser has 

 a cone of 12 feet in height and a diameter of 20 feet on top and 120 feet 

 circumference at the base. This cone is on a platform that measures 

 75 by 100 feet and is 3 feet in height, and the whole thing — cone and 

 platform — is on a large mound of deposits that rise 40 feet above the 

 level of the river and cover about 3^ acres. 



The deiDOsit in the mound on which the Giantess, Bee Hive, and Trinity 

 Geysers are located occupies 22 acres, but it is of unequal thickness. The 

 Trinity Geysers are 43 feet above the river level. At the Bee Hive the 

 elevation is less, and at the Giantess more. The latter geyser has a 

 depth of 63 feet. The table on page 369 gives the areas of other groups 

 and shows the comparison with those of Iceland and New Zealand. We 

 will now consider the rapidity of the deposition of these sediments. 



RAPIDITY OF DEPOSITION. 



Calcareous tufa. — As we have just noted, the deposits of calcareous 

 tufa or travertine usually exceed in quantity the siliceous sediments. 

 This is due mainly to the greater rapidity with which the former are 

 laid down. 



At the baths of San Vignone, in Italy, 6 inches of tufa is deposited 

 yearly in the conduit pipes. At San Fillijto, in 20 years, a bed 30 feet 

 thick has been formed in a pond. 



At the Mammoth Hot Springs of Gardiner's River, in the Yellow- 

 stone Park, baskets and other articles of wire are placed in the over- 

 flow from Cleopatra Spring, and are coated with calcareous deposit. 

 About one-sixteenth of an inch is deposited in four days under favor- 

 able circumstances. It would be impossible, however, to compute from 

 this the age of the deposits at this locality, for under other circum- 

 stances the rate is much lower, and, again, it may frequently be more 

 rapid in some other places. 



Siliceous sinter or geyserite. — Forbes, who visited the Great Geyser of 

 Iceland in I860, jilaced a bunch of grass under a small fall where the 

 ejected mass of water from the geyser drained rapidly away. He found 

 that in tweiiity-four hours it received a coating of silica of about the sub- 

 stance of a very thin sheet of paper. He then made a calculation that, 

 allowing 500 such sheets of paper to the inch, Avith the tube having 

 a length of 762 inches, the age of the geyser would be 1,036 years. 

 This calculation might be all right if the deposition took place regu- 

 larly. But, as we shall see presently, the deposit occurs mainly when 

 the water is allowed to evaporate, and this will occur most rapidly 

 where the water rises and falls alternately, or when it frequently over- 

 flows from the basin. A reference to Fig. 28 (the figure explaining 

 McKenzie's theory of the geyser) will help make this clear. This 

 illustration gives a section of the Great Geyser. As the basin Pis 

 usually full, the evaporation will be more constant around the edges, 

 which tends to build up the edge constantly. Whenever there is an 



* John Coles gives the following for the size of the cone of the Great Geyser in 1881 : 

 12 feet above the general level and 101 yards by 75 yards at the base. (See Summer 

 Travelling in Iceland, &c. London, IS&i.) 



