413 



concrete of PLOMBIERES render it almost certain that thermal waters are 

 the agencies by which these minerals are formed in rocks. These springs 

 issue from fissures in the granite, which fissures stand in relation to the 

 mineral veins of the district. The temperature of the warmest of them 

 does not exceed 70. Those which have a temperature between 15 and 

 32 deposit a hydrous silicate of alumina allied to halloysite. The 

 quantity of matter dissolved in these springs does not exceed 

 30 milligrammes per litre. Silicate of potash is the dominating 

 constituent and fluoride of calcium also occurs. The waters were con- 

 ducted by the Romans in channels formed of a concrete composed 

 of fragments of brick and sandstone cemented by lime unmixed with 

 sand. In the course of ages various minerals have been formed in 

 the concrete, including apophyllite, chabasite, chriastianite, hyalite, opal, 

 fluorite, calcite, arragonite, a hydrous magnesium carbonate and halloy- 

 site. Apophyllite, a non-aluminous zeolite containing calcium and 

 potassium together with a small quantity of fluorine, is found only 

 in the cement ; chabasite, on the other hand, an aluminous zeolite, 

 is found in the brick. This localization of the zeolites shows that 

 they are formed in place by a re-action between the alkaline waters 

 and the different constituents of the concrete. In the development 

 of zeolites and the deposition of silica in this concrete by the action 

 of thermal waters we evidently have illustrations of a process which 

 must take place extensively in many volcanic districts. The basaltic 

 rocks of the North of Ireland and West of Scotland are in many 

 places charged with zeolites which probably owe their origin to causes 

 similar to those above referred to. 



We have now to refer to what may be called the normal weather- 

 ing of igneous rocks. When subjected to the physical and chemical 

 conditions which prevail at the surface of the earth igneous rocks are 

 in a state of unstable equilibrium. They are liable to undergo de- 

 composition and disintegration. The different constituents are of course 

 very differently affected, but in order that a rock may be completely 

 disintegrated it is only necessary that one of its principal constituents 

 should be liable to decomposition. Of all rock-forming minerals the 

 felspars are perhaps the most important. These are liable to undergo 

 decomposition under the influence of surface agencies. The Brothers 

 ROGERS, and many later observers, have shown that water charged with 

 carbonic acid will decompose orthoclase and remove alkali and silica, 

 together with a small quantity of alumina. DAUBREE has proved that 

 prolonged trituration of orthoclase in pure water is accompanied by a 

 decomposition of the mineral ; the water taking up potassium silicate 

 and becoming markedly alkaline. The weathering of felspar is accom- 

 panied by a diminution in the amount of the alkalies, alkaline earths 

 and silica, and by an increase in the amount of water. The ultimate 

 residual product in all cases is kaolin. In the lime-soda felspars car- 

 bonate of lime may be formed and when this is the case the rock will 

 of course effervesce with acids. The process of decomposition above 



