Lawson.] 



Malignite, 



combined in the silicates, there being but mere traces of iron ores. 

 Both iron and magnesia are, on the other hand, more abundant 

 than is usual in the alkali-rich plutonic rocks. Mineralogicallv, 

 their constant characteristic is the prominence of orthoclase, with 

 which is often associated acid plagioclase in microscopic intergrowth. 

 Quartz is wholly absent. The constant ferro-magnesian silicate is 

 aegerineaugitc, which may predominate, with but a moderate 

 admixture of biotite, or may be subordinate and intergrown with a 

 preponderant soda-amphibole, biotite being present as before. In 

 one of the three types of malignite, melanite is an essential and very 

 prominent constituent. In another, nepheline enters into the com- 

 position of the rock. 



Such alkali-lime-rich rocks with low silica and moderate alum- 

 ina, iron, and magnesia, find no place in the existing families of 

 plutonic rocks. They are clearly not granites. The}' are too low 

 in silica and too high in lime, and rather high in alkalies for sye- 

 nites, if that term is to have any precision of significance. They 

 have certain affinities with the nepheline-syenites ; but here again 

 their characteristically high lime contents, and in a less degree their 

 low silica, bars the way to their admission to the family. Besides, 

 Olily one of the three types contains nepheline, and that is petro- 

 graphically, as well as geologically, much more closely affiliated 

 with the other two which have no nepheline than it is with the 

 nepheline-syenites. Neither are they to be placed in Brogger's 

 newly-established family of the monzonites, since they are explic- 

 itly excluded by that author's definition of the characteristics of the 

 family,* although the latter is a generously hospitable one for many 

 a stray rock of hitherto dubious pedigree. Being orthoclase rocks, 

 the_\ r belong neither to the theralites nor the diorites nor the gab- 

 bros. How then shall we class them? Squeeze them in some- 

 where in an existing family, no matter how it vitiate the character- 

 istics of the family? Such has been a too prevalent custom, and 

 the process has gone on til! some of our rock families have become 

 unduly distended with foreign elements, each succeeding one more 

 remote from the type than its predecessor, and relief is effected 

 only by a violent disgorgement. By such a process of disgorgement 



* Die Eruptionsfolge bei Predazzo, pp. 53, 54. 



