OCR Text |
Show 112 GEOLOGY OF THE HIGH PLATEAUS. The foregoing scheme of classification is in the following conspectus given as a whole. Of the various sub-groups the following have not yet been detected among the eruptives of the High Plateaus: nevadite, por-phyritic trachyte, augitic propylite, dacite, nephelin-dolerite, leucite-basalt, nephelin-basalt. All of the others are well represented. The trachytic group, however, very far overshadows all the others in volume and variety. Group I.â€"Acid rocks. Ehyolites. Sub-groups. 1. Nevadite. 2. Liparite. 3. Ehyolite (proper). Group II.â€"Sub-acid rocks. Trachytes. Sub-group A.â€"Sanidin trachytes. Sub-group B.â€"Hornblendic trachytes. 1. Granitoid trachyte. 5. Hornblendic trachyte. 2. Porphyritic trachyte. 6. Augitic trachyte. 3. Argilloid trachyte. 7. Phonolite. 4. Hyaline trachyte. 8. Trachytic obsidian. Group III.â€"Sub-basic rocks. Andesites. Sub-groups. 1. Hornblendic propylite. 4. Hornblendic andesite. 2. Augitic propylite (?). 5. Augitic andesite. 3. Quartz propylite. 6. Dacite. Group IV.â€"Basic rocks. Basalts. Sub-groups. 1. Dolerite. 4. Leucite basalt. 2. Nephelin dolerite. 5. Nephelin basalt. 3. Basalt (proper). 6. Tachylite. |