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I. Fine grained norite from the Tinnebach. 

 II. Typical norite from Oberhofer. 

 III. Quart-norite from the Vildarthal. 

 IV. Quartz -norite between Johannser and Muttlur Hof. 

 V. Quartz-mica-diorite from the Vildarthal. 



The first four rocks may be regarded as typical enstatite-diorites or 

 rather as typical enstatite-augite-diorites. We have described them some- 

 what fully because they closely resemble the " diorite " or " enstatite- 

 diabase" of Penmaenmawr and because they must be regarded as the 

 hypogene representatives of such rocks as the Cheviot enstatite-augitc- 

 andesites. 



Andesite. This name was introduced by L. VON BUCH for rocks which 

 had been previously classed with the trachytes under the impression that the 

 dominant felspar was sanidine. Like almost all other names in petrography it 

 has been so variously used by different writers that it has almost ceased to 

 have any definite significance. If Ave define andesite as the volcanic repre- 

 sentative of the intermediate magma we shall find that it will correspond 

 very closely with the common use of the term. 



The lavas of the Andes furnish typical examples of andesites, as indeed 

 the name implies. Precisely similar rocks are found all round the Pacific 

 " circle of fire," in Central America, the west of North America, Japan and 

 the islands of the East Indian Archipelago. 



The andesites are distinguished from the trachytes which they 

 requently resemble in general aspect by the predominance of a plagioclase- 

 felspar labradorite, andesine or oligoclase. The so-called oligoclase- 

 sanidine trachytes may be regarded as intermediate between andesites and 

 trachytes. The majority of the andesites are porphyritic rocks in which 

 both felspar and a ferro-magnesian constituent occur as porphyritic elements. 



In some rocks which have been termed andesites original quartz also 

 occurs. These are as a rule acid rocks and will be described in the present 

 work under the general term dacite. The quartz-free andesites may be 

 conveniently subdivided into groups according to the nature of the ferro- 

 magnesian constituent. We thus have : 



AUGITE-ANDESITE. 

 ENSTATITE-ANDESITE, 

 HORNBLENDE-ANDESITK. 

 MIC A- ANDESITE. 



In most cases more than one ferro-magnesian constituent is present. 

 These can, however, be always designated by using compound names such 

 as enstatite-augite-andesite. 



Dr. SZABO does not recognize the term andesite. He uses the general 

 term trachyte to cover not only the orthoclase-bearing lavas but also those 

 in which plagioclase is the dominating felspar. 



When the porphyritic plagioclase has been determined its name may 

 also be introduced. We may thus build up compound terms which are in 

 reality concise descriptions of the rocks. In the vast majority of cases the 

 rocks termed andesite are distinctly porphyritic. This rule is so general that 



