84 



In order to explain the causes wliich have led to the deposition 

 of Calc-tufa at Chalford and at Dursley, and why in those 

 localities the waters of the springs so soon part with their 

 recently acquired Carbonate of Lime, it is well to study the like 

 results in some small spring— as, for instance, one near Nailsworth, 

 which is thrown out by the Upper Lias, and whose waters, being 

 derived from the rainfall on an extended slope of the Inferior 

 Oolite, are charged with Carbonate of Lime. The water from this 

 spring passes over a series of miniature falls, at the first of which 

 the water runs down stones, and there a deposit of Tufa takes 

 place on the stones over which the water trickles. The next is 

 a clear fall from the edge of a slab of stone on to another slab 

 placed three feet below, and then again to a pool. Round this 

 fall Hypnum commutatum, the " Curled Fern Feather-moss," is 

 growing. This moss, which has its specific name from its being 

 frequently found so incrusted with calcareous matter as to appear 

 "changed" into stone, delights in moisture, and grows so freely 

 in and around a waterfall, that though part of a plant may be thus 

 apparently changed into stone ; its young shoots grow on, and 

 ever present fresh scafiblding for the support of the increasing 

 mass of Tufa. Other species of mosses and liverworts grow in 

 such a situation, and aid in arresting the particles of lime as the 

 water passes over them. Leaves of plants, numerous insects, 

 shells of land and fresh-water molluscs, become imbedded in the 

 growing mass of Calc-tufa, and by their "organic remains" 

 attest the recent date of the " formation." 



The rate of increase of a mass of Tufa must necessarily vary 

 with circumstances, but in the fall above mentioned the growing 

 plants of Hypnum commutatum have become " changed " to the 

 extent of one yard in depth during the last year. 



By observing the mode of deposition of Tufa in a small stream, 

 and finding that it is when the waters containing Carbonate of 

 Lime are spread out in a thin stratum so as to permit escape of 

 the carbonic atjid and evaporation of the water that a deposit 

 of Tufa takes place — and to obtain this a fall of the waters is 

 required — it is at once seen that such conditions must have 

 prevailed to cause the large deposits at Chalford and Dursley. 



