178 prop. e. j. gaewood on the [May 1906, 



The solubility of the rock is shown by its crumbly and cavernous 

 character where it is exposed at the surface (see PI. XII, fig. 1), 

 and by the fact, mentioned above, that the surplus-water from 

 Lago Tom, instead of finding an exit over the rauchwacke, dissolves 

 its way underground and reappears exactly at the junction of the 

 rauchwacke with the underlying insoluble schist. 



Experiments prove that the rock is rapidly disintegrated by 

 water containing carbon-dioxide in solution. Miss Goodyear 

 finds : — 



Per cent. 



Soluble in cold dilute hydrochloric acid 75'8 



Soluble in hot dilute hydrochloric acid 92 



Soluble in H 2 4- C0 2 11 in 45 hours. 



Assuming the difference between the solubility in hot and cold 

 acid to be due to dolomite, the composition of the rock would be as 

 follows : — 



CaCO, 73-4 



Dolomite 162 



Fe 2 3 1-3 



Insoluble residue 8 - l 



99-0 



Microscopic examination shows how this solubility is assisted 

 by the granular structure of the rock, which readily falls to pieces 

 so soon as the crystals of calcite are dissolved. 



Being impressed, when sounding these lakes, with the possibility 

 of their having originated from solution of the rauchwacke, I 

 dredged several samples of the deposits from the floor of Lago 

 Tom and Lago Cadagno. These were taken from their southern 

 sides, where the deposit overlies the outcrop of the rauchwacke. 

 They consist, in both cases, of coarse fragments of various minerals 

 mixed with a fine white micaceous sand. The coarser fragments 

 are composed of quartz, garnet, actinolite, and mica, derived 

 from the crystalline schists above, and washed into the lakes by 

 streams from the north. The finer material, after sifting through 

 a sieve of 80 meshes to the inch, consisted of little rounded grey 

 grains, together with a few flakes of mica, etc. On boiling in 

 dilute acid, the former dissolve readily, and prove to be dolomite- 

 grains, which have fallen on to the floor of the lake after the 

 solution of the calcite and the disintegration of the rauchwacke. 

 This finer material, therefore, represents the less-easily soluble 

 residue of the rauchwacke. The collection of the dolomite-grains 

 into one sieve is due to the wonderfully-uniform grain of the 

 recrystallized rock. With the view of further testing the solubility 

 of this rock, I collected samples of the water from Lago Tom 

 and Lago Cadagno. 1 The samples were collected where the water 



1 Taken after a week of very heavy rain. 



