218 UNSTRATIFIED OR IGNEOUS ROCKS. 



mineralogical composition and in crystalline structure, and is therefore 

 very probably true. We have substantially assumed it in the preceding 

 descriptions. 



The confusion in the classification and nomenclature of igneous 

 rocks is still further increased by the undoubted fact that many of the 

 kinds of rocks mentioned above as igneous are found also among met- 

 amorphic rocks which have never been erupted at all. This subject is 

 further treated under the head of Metamorphism (p. 223 et seq.). 



3. Riclitliofen 's Classification of Tertiary Eruptives. 



By far the most successful attempt to classify by age, or to corre- 

 late the kinds of igneous rocks with their ages, is found in Richt- 

 hofen's classification of Tertiary eruptives. According to Richthofen, 

 there is a regular and invariable order of succession among the erupt- 

 ive rocks of Tertiary times ; the order being — 1. Propylite.* 2. Ande- 

 site. 3. Trachyte. 4. Rhyolite. 5. Basalt. This order, however, ap- 

 plies only to primary or fissure eruptions ; for, since primary erupted 

 masses may become the seats of subsequent secondary or crater erup- 

 tions, it is evident that secondary eruptions of a lower group may be 

 synchronous with primary eruptions of a higher group, f 



These views of Richthofen's have attracted wide attention, but have 

 not been generally confirmed. All that is as yet universally accepted 

 in regard to the order of Tertiary eruptives is that the trachytes (in- 

 cluding in this term with the trachytes proper also the andesites and 

 the rhyolites) precede the basalts. The reason of this may possibly be 

 found in the fact that acidic rocks, although more infusible than the 

 basics to dry lieat, yet yield very easily to hydrotliermal fusion by the 

 formation of hydrous silicates. Now, it is in this condition of imper- 

 fect hydro thermal fusion that the trachytes and rhyolites were erupted, 

 while the basalts have been in a state of complete igneous fusion. If 

 we suppose strata of different kinds to be subjected to steadily increas- 

 ing heat in the presence of a small percentage of water, it is easily con- 

 ceivable that the acidic rocks would first yield by hydrothermal fusion, 

 and only afterward the basic rocks by true igneous fusion. 



Judd, in his recent work on Volcanoes, admits that an intermediate 

 type like andesite (propylite is usually regarded as a variety) is first 

 erupted, then an acid type like trachyte and rhyolite, and last basalt. 

 He accounts for this by supposing a homogeneous fused mass (such as 

 would be formed by fusion of many different kinds of strata), to be 

 first erupted as soon as formed. This would make an intermediate 



* Propylite is regarded by many as an altered andesite. 



f Richthofen's Natural History of Volcanic Rocks, Memoirs of California Academy of 

 Science, vol. i, Part II. 



