172 E. ABBAY ON THE BUILDING-UP OF THE 



of the more strikingly typical terrace-basins. These, as has been 

 said, are roughly semicircular and in appearance not unlike huge 

 shells arranged with their edges horizontal. The wall of one basin 

 slightly overhangs the water in the next, as will be seen by a re- 

 ference to fig. 1, in which a b represents the overhanging wall, a 

 being the upper edge of it ; c is a somewhat flocculent deposit of 

 silica left by the water that streams down a b ; dis the main body of 

 water within the basin ; and m represents a stratum of water from 

 6 to 12 inches in depth flowing towards the lip, and of a con- 

 siderably higher temperature than the mass of the water below it. 

 The deepest part of the basin is generally about the middle ; but this 

 necessarily varies according as the water flowing into it comes from 

 one or more basins, the deposit of silica c depending in shape, of 

 course, on the water coming from these basins. On carefully ex- 

 amining the outer wall b, it is found to consist of a number of ap- 

 proximately horizontal ledges or minute basins (fig. 2), each of 



Eig. 2. — Block of Sinter. 



Half natural size. 



Showing the outer surface, with ridges forming miniature basins (a, a, a), and a 

 fractured surface with parallel surfaces of deposition (b, b, b). 



these apparently corresponding in miniature to the larger basin. It 

 is evident that these are due to the deposition of silica on the edges 

 as at a a a, and to the tendency to a greater growth at these points 

 than elsewhere. A transverse section shows the parallel surfaces 

 within the mass (b b 5), thus proving that this system of deposition 

 is regular. The above facts suggest to ns what the process of 

 building up these terrace-basins is, and what is the general effect 

 on the whole mass of the sinter-formation. In the first place, the 

 hot water flowing from m towards a (fig. 1) begins to be suddenly 

 cooled on reaching a, and still more so on flowing down a b ; for it 



