330 ORIGIN OF ERUPTIVE AND PRIMARY HOCKS. 



layers. Sedimentary rocks occur only in the latter form. Erup- 

 tive rocks are totally destitute of fossils and their ages are deter- 

 mined by the relations of contact, -which exist between them and 

 sedimentary rocks. Fossils constantly occur in the latter, and con- 

 stitute the principal means of determining their age. Eruptive 

 Tocks resemble in the mode of their formation the slags, which 

 run out of smelting furnaces • sedimentary rocks the slimes depos- 

 ited in stampworks and allowed to consolidate. 



The eruptive formations have been arranged in the order ©f 

 theiy antiquity by Naumann as follows : 



1. The granulite formation. 



2. The granite do. 

 S. The syenite do. 



4. The greenstone do. 



5. The porphyry do. 



6. The melaphyr do. 



7. The trachyte do. 



8. The basaltic do. 



9. The lava do. 



This arrangement is however general and approximative. No& 

 only do the rocks of these formations in their lithological charac- 

 ter graduate into each other, but the latter part of one formation 

 may have been erupted simultaneously with the earlier rocks of 

 the succeeding one. Thus trachytes and basalts are almost of 

 contemporaneous origin, and porphyries have been protruded 

 through the earth's crust in the same periods as certain greenstones 

 and melaphyrs. I shall therefore class several of these formations 

 together and refer to them in the following order: 



1. Trachyte, Basalt, and Lava. The volcanic formations of 

 Naumann. 



2. Phorphyry, greenstone and melaphyr, ) The plutonic forma- 



3. Granite, syenite, and granulite, | tions of Naumann. 

 Trachyte, Basalt, and Lava. I have already adverted to the 



distribution of volcanoes as constituting a proof of the existence 

 of a molten zone betwixt the central metallic globe and the crust 

 of the earth. I do not deem it necessary to enlarge much upon 

 this point. As Naumann remarks: " Volcanoes exist in every 

 part of the earth, under every latitude, under the equator and near 

 to the poles, in the torrid as well as in the temperate and frigid 

 zones. They are confined to no climate, because in Iceland., 



