318 DESCRIPTIVE GEOLOGY. 



Snake Creek, just above the village of Medway, is a group of warm springs, 

 the most important and considerable observed in this region, and which are 

 of interest in connection with the trachyte body which they adjoin. They 

 issue from conical or dome-shaped mounds, and deposit a great amount of 

 calcareous tufa ; many of the cones or mounds are now dry and choked up, 

 or covered with soil and vegetation. The tufa deposits, which form a sort 

 of low plateau, cover a very considerable area, probably 2 or 3 square 

 miles, and over this plateau are dotted the active and extinct springs, vary- 

 insr in size from small orifices a few inches in diameter to cones and mounds 

 built up 20 or 30, and even as much as 60 feet above the general level. 

 Most of the extinct as well as active springs have the shape of a hollow 

 truncated cone, the interior cavity being wider at the base than near the 

 rim. — that is, the tufa, which deposits on the rim as the water cools in contact 

 with the atmosphere, builds up the inside of the enclosing crust faster 

 than the outside; thus the orifice is continually closing together. Probably 

 in the same way the lower channel, through which the water rises from 

 below the surface, has become completely choked up in the case of the 

 numerous extinct or dry cones. One of these may be seen in Plate XXI, 

 Chapter V, which represents a dry tufa cone about 15 feet inheight, formed 

 of layers of calcareous tufa having a rudely concentric structure. The 

 interior cavity, which has been partly filled by soil and detritus, is 3 or 4 

 feet in diameter, and nearly 5 feet deep. It is less regular in shape than 

 some of the active springs, its material yielding readily to atmospheric 

 disintegration. The active springs are, however, more generally of a flat 

 dome-shape, with a truncated top. Of these, the most perfect one observed 

 is a mound about half a mile north of the town of Medway, rising proba- 

 bly 10 feet above the general surface. Its orifice is an almost perfect circle, 

 20 feet in diameter, filled to within a couple of inches of the brim with clear 

 tepid water, having a temperature of 85° to 90° : from it is a slight seepage 

 of water, through a little break in the otherwise perfect rim. The water is 

 perfectly clear, aiid the bottom, which was apparently about 15 feet below 

 the surface, could be seen with perfect distinctness ; the diameter increases 

 so rapidly in depth that a person holding himself vertically, at arm's length, 

 with his hands on the rim, could scarcely touch the interior walls with his 



