216 



UXSTRATIFIED OR IGNEOUS ROCKS. 



sorting, and deposit, throughout all geological times, has been to pro- 

 duce extreme differentiation of stratified rocks, belonging to the same 

 time and in the same region. Hence, if eruptives are produced by re- 

 fusion of these, we would expect to find great diversity among them. 



But, on the other hand, the extreme diversity which we find among 

 stratified rocks, viz., pure sandstones (acid), on the one hand, and pure 

 limestones (base) on the other, is not found among igneous rocks. But 

 this, which seems at first an objection, is found, on examination, a con- 

 firmation of our conclusion; for these extremes are very difficult of fu- 

 sion. Thus the diversity of composition of igneous rocks is completely 

 explained by supposing them formed by refusion of stratified rocks 

 within the limits of ready fusibility.* 



2. Other Modes of Classification. 



There is no subject connected with geology which is in a state of 

 greater confusion than the classification and nomenclature of igneous 

 rocks. It seems proper, therefore, to mention some of the different 

 views entertained. 



Many geologists think that igneous rocks may be thrown into three 

 groups, characteristic of different periods of the earth's history, and 

 which, therefore, are now found associated with the stratified rocks of 

 different ages. These are : 1. The granitic group, including granites 

 and syenites, associated with archaean and palaeozoic rocks; 2. The 

 trappean group, including diorites, porphyry, dolerite, etc., associated 



with the later palaeozoic and the mesozoic 



rocks ; and, 3. The volcanic rocks, including 

 basalts, trachytes, etc., associated with the 

 tertiary rocks. They think, therefore, that 

 the earliest eruptions were granitic, then 

 trappean, and lastly volcanic. Furthermore, 

 they think that the first have come up most- 

 ly in great, dome-like masses ; the second, 

 mostly intrusive, in dikes and fissures ; and 

 the third through craters forming volcanoes. 

 Again, many think that erupted matters, 

 of different times, have become progressively 

 more basic. They think that, although each 

 group may be divided into a more acidic and 

 a more basic sub-group, yet, as a whole, the 

 granitic group is the most acidic and the 

 volcanic the most basic, the trappean being intermediate, as shown in 

 the accompanying diagram. 



Basic. 



Acidic. 



Basic. 



Volcanic. 



Trachyte. 



Basalt. 



Tra-ppean. 



Porphyry. 



Diorite. 



Granitic. 



Granite. 



Syenite. 



Acid. 



* Captain Dutton, High Plateaus of Utah, p. 120. 



