MINERAL VARIETIES OF ANDESITE. 239 



valent of diorite ; and though mica is more characteristic of diorite 

 than of andesite, most of the varieties of diorite may be paralleled by 

 varieties of this rock. When andesites are decomposed their percent- 

 age of silica is much reduced. Koch divides the Hungarian andesites 

 into seven varieties, of which two are classed by Eosenbusch as augite- 

 andesite, and five as hornblende-andesite. 



The latter comprise (i.) Labrador-biotite-garnet andesite: (2.) 

 Labrador - biotite - garnet - augite andesite ; (3. ) Labrador - amphibole 

 andesite; (4.) Labrador-amphibole-augite andesite; and (5.) Labra- 

 dor-amphibole-biotite andesite. The minerals thus named occur as 

 crystals in a ground mass, which may be clear as glass, or opaque 

 with crystallites, or rich in orthoclase, as in (i) and (2). 



Concretionary Inclusions in Andesite. The hornblende andesites 

 of the Siebengebirge are remarkable for the concretions and small 

 angular inclusions which they contain. The concretions are well de- 

 veloped at Great and Little Eosenau, and in the Stenzelberg, where they 

 are known to the quarrymen as black granite. They vary from an inch 

 or two in size to a diameter of many inches, and consist of conspicuous 

 columns of hornblende imbedded in a ground mass of andesite. The 

 size of the hornblende crystals and the amount of ground mass be- 

 tween them both vary. When the proportion of hornblende is small, 

 the external shape of the concretion may be undefined, but when the 

 hornblende crystals are in contact, and include the matrix between 

 them, then the external form of the concretion is that of hornblende, 

 as well defined as though the matrix were limited to the centre. 

 Large porphyritic crystals of compact hornblende occur. The crystals 

 appear to have grown gradually at the expense of the surrounding 

 rock, the large forms being built up by the gradual increase in size 

 and blending of many small crystals. In the augite-andesite of the 

 Lowenburg we find similar concretions, only the section is octagonal, 

 and therefore presumably determined by the augite. The texture 

 of these concretions is often the finest possible. The separation 

 between the concretion and the surrounding rock is clean and easy, as 

 though a slight film of kaolin parted them. These concretions are 

 not unlike the mica concretions in granite already described, which 

 may have had a like origin. At Little Kosenau silica sometimes 

 separates so as to form quartz veins in the andesite; the ground 

 mass is cryptocrystalline. It contains, with crystals of plagioclase 

 and oligoclase, much hornblende ; but sometimes biotite and augite 

 replace hornblende. The percentage of silica varies from 46 to 51. 



Mineral Condition of Andesite. The plagioclase is usually 

 microtine in the glassy varieties of the rock ; but in some dacites statf- 

 like crystals of triclinic felspars are frequently aggregated into com- 

 pound crystals ; and it may be that the plagioclase is partly labrado- 

 rite and partly oligoclase, with some other felspars. The crystals con- 

 tain inclusions of the base, inclusions of glass and steam cavities, 

 microliths, and occasional fluid cavities. In Hungary and Transyl- 

 vania fluid cavities abound in dacite, and are absent in andesite. The 

 crystals sometimes are made of fragments united together, some- 



