INDIAN CREEK LACCOLITH. 63 



micropoikilitic quartz patches grow less and less noticeable, and the feldspar 

 microlites become more pronounced, the microstructure being more like 

 the characteristic felt-like or pilotaxitic structure of andesites. This transi- 

 tion occurs as the rock approaches the contact with inclosing rocks, and 

 where the body thins out. It accompanies a darkening of the rock and an 

 increasingly andesitic habit. The rock 20 feet from the bottom contact 

 near the east edge of the laccolith north of Indian Creek is bluish gray, 

 with prominent feldspars that are decomposed (60). The hornblendes are 

 altered to chlorite and calcite. Biotite is still fresh. The groundmass is 

 holocrystalline and pilotaxitic. Near the bottom contact of the middle 

 portion of the main laccolith body the rock grows darker and denser. At 6 

 feet from the contact it is darker gray than the main mass. The micro- 

 structure of the groundmass is micropoikilitic, with the minute feldspars 

 maintaining a fluidal arrangement (61). The rock 1 foot from contact is 

 darker colored; its structure is still micropoikilitic, with more minute feld- 

 spars (62), while the rock directly in contact with the limestone is still 

 darker and the microstructure still finer grained and micropoikilitic (63). 

 There is considerable calcite scattered in irregular microscopic ag-o-rea'ates 

 through the groundmass. The hornblendes are decomposed, and there is 

 some secondary quartz. The micropoikilitic structure, however, is not 

 secondary, as seems to be the case in some porphyries, 1 since it varies in 

 size of grain according to the distance from the contact plane, and is quite 

 the same as that observed in perfectly fresh andesite-porphyries in other 

 places. Similar modifications occur near the contact of the andesite-porphyry 

 with the inclosed belt of limestone in the central part of the laccolith north 

 of Indian Creek {&&)■ The feldspar and biotite phenocrysts are fresh, 

 while the hornblende is entirely decomposed. 



Where the andesite-porphyry is exposed in contact with the overlying 

 limestone at the northwest base of Three River Peak, the same transition 

 from coarser-grained to finer-grained groundmass is observed (68, 69, 70, 

 71). The rock nearest the contact is very dark colored, dense, and dis- 

 tinctly porphyritic, and under the microscope is found to have the micro- 

 structure of a holocrystalline andesite — that is, the groundmass consists of 

 microlites of feldspar and pyroxene, with scattered grains of magnetite, and 



'Williams, G. H., on the use of the terms poikilitic and inicropoikilitic in petrography: Jour. 

 Geol., Vol. I, No. 2, 1893. p. 179. Baseom, l<\, The structures, origin, and nomenclature of the acid 

 volcanic rocks of South Mountain : Jour. Geol., Vol. I, No. 8, 1893, p. 814. 



