202 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



ent day, but the knowledge of the chemical changes effected in various 

 compounds by heat justifies the assertion. In spite of the common origin, 

 as to this one factor, the differing character of the original unfused ma- 

 terial, and more particularly the results produced by various methods 

 of cooling, have supplied us with so many specifically distinct occur- 

 rences. 



In speaking of the origin of eruptives these features will be more thor 

 oughly discussed and instances referred to where they directly apply 



CLASSIFICATION. 



At all times it is a matter of difficulty to arrange any satisfactory 

 lithological classification. This is more particularly the case when we 

 deal with eruptive rocks. The means at our disposal whereupon to base 

 any definite system are limited, and in several instances unsatisfactory. 

 At first sight a chemical basis appears very acceptable, not only for the 

 primary, but also for minor divisions. It is, however, open to the 

 objection that in many cases it would be requisite to make analyses be- 

 fore being able to decide as to the species of rock under consideration. 

 Among the best systems offered is that used by Cotta.* He accepts 

 Bunsen's primary division into basic and acidic rocks, and determines 

 the species of each group in accordance with its mineralogical constitu- 

 tion. This method furnishes the means, first, of a definite ultimate 

 arrangement, and, secondly, of determining at once the precise position 

 occupied by the specimen under examination. 



It would seem that a classification based upon the geological age of 

 the rocks might prove serviceable. One main and decided objection, 

 however, asserts itself in the difficulty of determining this age in 

 many instances. Were we in the possession of all the necessary data 

 a system could, no doubt, be constructed that would answer admirably 

 well. It would necessitate the distinction into species utterly irrespect- 

 ive of genetic or mineralogical affinities, however, and probably create 

 more confusion than benefit. Of late years the microscope has been of 

 great service in determining lithological species, more particularly those 

 belonging to the eruptive class. Under the skillful management of 

 Zirkel this method has achieved a reputation justly earned by its excel- 

 lent results. In spite of its correct interpretation and by far more accu- 

 rate discrimination than any other means will supply, its very general 

 application for classificatory purposes will remain limited. The prepa- 

 ration of the rocks before the examination can take place, the transpor- 

 tation of instruments, and the comparatively large j?aiount of practice 

 required for the successful completion of work will prevent this method 

 from becoming the basis for a universally accepted classification of 

 rocks. 



Of all the means at hand, therefore, the one adopted by Cotta an- 

 swers best for our purposes, and it is with special reference to his gen- 

 eral system of classifying (without adhering strictly to his arrangement) 

 that the rocks in question shall be treated of below. 



Europe exhibits a number of typical localities where volcanic — in 

 contradistinction to plutonic — rocks occur. It is a few only of them 

 that enter into consideration here. The generally- adopted application 

 of names to European lithological groups is conformable with our own, 

 with the exception that our groups are by far more varied ; hence di- 

 vision and subdivision more thoroughly carried out. There, as here 

 (speaking of Colorado more especially), the trachytic and basaltic 



* Geologie der Gegenwart, Leipzig, 1872, p. 20. 



