A Geoscience Digital Image Library
A Geoscience Digital Image Library
| Title | Dacite, from northwest of Helena, Montana |
|---|---|
| Description | This is a gray porphyritic extrusive igneous rock with phenocrysts of gray plagioclase. It also contains small to medium size black hornblende crystals. The matrix consists chiefly of a very fine-grained quartz-feldspar intergrowth and scattered patches of wispy biotite. This sample is about 9 cm across. |
| Chronostratigraphy | Late Cretaceous |
| Location | USA ▹ Montana. Near Helena. |
| Photographer | Twyla Baker. 2001-02-15. |
| Collection | Ward’s Collection of Classic North American Rocks #26. |
| Key words | dacite, Montana |
| Tech details | 966 KB. Hand specimen. Fujifilm FinePix S1Pro digital camera; 60mm AF Nikon micro lens. |
| GeoDIL number | 90 |
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