190 Proceedings of the Royal Physical Society. 



of less common occurrence, such as Finite, etc. Under this 

 category must also be placed Agalmatolite, which appears 

 to stand in the same relation to Oligoclase as Kaolin does to 

 Orthoclase. Saussurite may, likewise, originate in the same 

 manner from Anorthite. 



{h) Those whose constituents have first undergone 

 solution within the lithosphere, and have sub- 

 sequently been redeposited at lower levels. 



This also is an important section, including as it does 

 the whole of the Zeolites and the minerals allied to them in 

 origin; and also Agates and various other forms of silica. 

 The rocks in connection with which the minerals under this 

 category occur are chiefly those of eruptive origin. The 

 number of mineral species whose origin may be classed under 

 this heading is considerable; but it will suffice for the 

 purpose in view in the present article to notice one or two 

 representative groups of each set. 



The prime factors concerned in the changes to which these 

 minerals are due appear to be water charged with Carbonic 

 Acid, which is principally derived from the atmosphere, and 

 with the allied Humus acids, which originate from the 

 decomposition of organic matter, chiefly through the action 

 of bacteria. In the case of an eruptive rock — of which we 

 may take an andesite as a typical example in the present 

 case — the weak solutions of acids percolating from the 

 surface begin by attacking certain of the constituents of the 

 i^ock — the sodium compounds amongst others. These are 

 sooner or later converted into solutions of sodium carbonate, 

 which, although very much diluted, are yet able to set up 

 important chemical reactions as they slowly make their way 

 downward through the body of the rock. One after another 

 of the constituent minerals of the rock begins to give way, 

 and the process is continued until, after a considerable length 

 of time, all the outer crust of the rock is completely decom- 

 posed — a small part of the dissolved constituents being trans- 

 ferred in solution by the percolating waters to positions 

 within the sounder rock below; while rivers transport the 

 remainder in the direction of the sea. 



