58 CLASSIFICATION IN LITHOLOGY. — GENERAL CONCLUSIONS. 



rocks belonging to tlie species basalt, but in some cases the older forms of 

 andesite have been described as melaphyrs. So, too, the term diabase 

 includes under it, in the common artificial nomenclature, both old basaltic 

 and andesitic rocks — a fact which has been imperfectly recognized in the 

 division of the diabases into diabase and olivine-diabase. In this work 

 the term melaphyr and diabase will be confined to the rocks belonging 

 under basalt, which possess the general characters of the ordinary melaphyr 

 and diabase type. In the case of some names — like diorite, which has been 

 commonly used for a wide range of old and more highly altered rocks than 

 those to which the names melaphyr and diabase have been usually given — a 

 greater diflticulty occurs in practical use. Such names are placed as sub- 

 varietal names, after the names of the variety or species to which they 

 belong, in order to indicate the line of derivation. 



Many terms have been given in the past to rocks, indicating some special 

 stage in their alteration, which are not of sufficient importance to be used as 

 a variety or sub-variety form. These terms are sometimes inclosed in a 

 parenthesis and placed after the specific or variety name. It is necessary to 

 place those rocks, whose derivation is not clear, as nearly as j^ossible in their 

 apparently proper situation, leaving it to the future to correct and amend 

 the classification. 



The fragmental rocks are placed, so far as possible, under the rocks from 

 which they are derived ; but those whose derivation is unknown, it is in- 

 tended to describe and classify according to the common method of nomen- 

 clature, as the best now practicable. 



The recent or unaltered fragmental rocks will be placed as tufas under 

 their proper species, while the older and altered forms will be classed as 

 porodites,* under their respective species and varieties. 



The object is to show the natural relations of the rocks so far as possible, 

 and at the same time to interfere but little with the customary use of litho- 

 logical names. 



As illustrations of my meaning, the following examples may be given, 

 the label of a melaphyr would be written as follows: Basalt, Melaphyr — 

 the following term being always subordinate to the preceding one in this 

 order: species, variety, sub-variety, etc., while any one of the names can 

 be used in speaking of the rock. 



If a rock to which the name diorite would be applied in ordinary nomen- 



* Bull. Mus. Comp. Zo5l., 1879, v. 2S0. 



