U8 



orthoclase as a constituent of certain diabases in the Midland Valley of 

 Scotland. 



The alteration of the felspars of basic igneous rocks by the various 

 metamorphic agencies is a subject of great interest and importance. At the 

 same time it is one of considerable difficulty. This difficulty is due, partly to 

 the difficulty of determining the precise nature of the alteration products, 

 and partly to our ignorance of the conditions under which the various 

 products arise. The modification of the felspars by surface agencies gives rise, 

 under certain circumstances, to the formation of zeolites, of which prehnite, 

 natrolite and analcime are the most common. Under other circumstances, the 

 felspar is destroyed without the formation of zeolites, the ultimate product of 

 alteration being, in this case, a hydrated silicate of alumina from which the 

 lime has been wholly, and the soda and silica partially removed. Calcite and 

 quartz are frequently formed in veins and cavities in the rocks in connection 

 with this change. The decomposition of the felspar is of course usually 

 affected in conjunction with that of other minerals, so that the conditions 

 become complicated by the presence of solutions of various salts, and 

 new minerals, containing elements not present in the original felspar are thus 

 frequently formed. In this way the magnesian silicates, such as chlorite and 

 pseudophite, which frequently occur as pseudomorphs after felspar may be 

 accounted for. Professor PUMPELLY (1) has pointed out that, in the basic igneous 

 rocks, the formation of chloritic pseudomorphs after felspar is often preceded 

 by the devolpment of prehnite. The comparative insolubility of alumina 

 prevents, as a rule, the removal of this substance. Nevertheless, the presence 

 of the more powerful acids, and especially sulphuric acid, may give rise to the 

 formation of soluble compounds of this base. 



Of the various substances which arise in connection with the alteration 

 of the felspars of basic igneous rocks, the so-called saussurite is undoubtedly 

 one of the most important. The term saussurite was applied by SAUSSURE to a 

 tough, compact or fine grained light coloured substance, possessing the 

 hardness of quartz, and a specfic gravity of from 3*32 to 3*4, and which, 

 together with smaragdite, made up the rock for which HAUY proposed the 

 name euphotide. The name was introduced under the impression that the 

 substance was a definite mineral. Subsequent researches have shown that 

 this is not the case. At the same time the term is one which it is found 

 extremely convenient to retain. Many varieties of saussurite have been 

 analysed, with the general result that the substance is found to vary 

 considerably in composition. The silica ranges from 43 to 50 per cent. ; the 

 alumina from 25 to 32 per cent ; the lime from 10 to 18 per cent ; and the 

 soda from 1 to 5 or 6 per cent. Variable quantities of iron, magnesia and 

 water occur. Dr. STERRY HUNT investigated the chemical and physical 

 properties of the saussurite of certain Swiss gabbros or euphotides. He 

 described it as compact and tough, with a splintery fracture, and of a 

 whitish, greenish, bluish or reddish colour. It contained lamella of triclinic 

 felspar and actinolite. Specific Gravity 3 '3, Hardness 7. In chemical 



(1) On the metasomatic Development of the Copper-bearing rocks of Lake Superior. 

 Proc. Amer. Acad., 1878. p. 253. 



