44 CAT.IKORXIA AC.\l)I-:>rV OI-- SCIENCES. 



desine or labradorite. Quartz is sometimes present in 

 rounded and corroded grains, occasionally with sharp 

 crystallographic outlines ; it contains inclusions of fluid 

 gas and probably also of glass. Straight, brown biotite 

 foils and greenish brown hornblende in well-detined long 

 prisms of the usual section, often twinned and partly con- 

 verted into chloritic products, close the list of porphyritic 

 minerals. As accessories occur constantly zircon, apa- 

 tite, titanite and sometimes malacolite in small greenish 

 prisms. 



The groundmass is always holocrystalline, but may ha\'e 

 several structural forms: i. AUotriomorphic microcrys- 

 talline, consisting of quartz and usually unstriated feld- 

 spar. 2, Microcrystalline, with a structure somewhat re- 

 lated to the granophyric : each quartz grain, contains many 

 smaller feldspar grains with irregular orientation. I^oth 

 these structural I'orms may occur together in the same 

 specimen. In quite a number of specimens this structure 

 is prominent. 3, Lathlike-granular, composed of lathlike 

 plagioclase cr3'stals, between which lie irregular grains of 

 quartz and unstriated feldspar. From this latter form of 

 hornblende-andesites, in which quartz and orthoclase are 

 not present as phenocrysts, there is but a short step to a 

 iine grained hyphidiomorphic granular hornblende plagi- 

 oclase rock : if only one generation of lathlike or prismatic 

 plagioclase crystals is developed the rock will be hyphi- 

 diomorphic granular and a diorite. Both at Dearborn 

 Creek and in the Little Belt Mountains, such granular 

 rocks are represented and appear in so close connection 

 with the normal porphyritic forms that their geological 

 equivalence cannot be doubted. These diorites, appar- 

 ently analogous to those which Stelzner has called •• An- 

 dendiorite '" are usually fine to medium grained rocks in 

 which hornblende needles and feldspar prisms may be 



