492 THE DISTRIBUTION, ETC, OF ALKALINE ROCKS, 



light on the building of our continent and the broader features of 

 structural geology. 



Definitions. — By the term alkaline rock I understand any rock 

 which contains a particularly high percentage of combined alkalies 

 or which under the microscope contains a great abundance of 

 soda-rich and potash-rich minerals. The actual percentage of 

 alkali necessary to make a rock fall in this group depends on its 

 basicity. Usually the rocks classed as alkaline also contain a 

 particularly low percentage of MgO and CaO. These constituents, 

 however, become very high in some basic varieties of alkaline 

 rock, such as fourchite, camptonite and ouachitite, which are 

 classed hereon account of their constant association with alkaline 

 rocks. Potash-rich leucite rocks which show a deficiency in 

 certain common rockforming constituents I would also class as 

 alkaline, although richness in soda and sodic minerals is the main 

 feature of the group. 



In general, therefore, the term alkaline, as applied to rocks, 

 will be used in the same sense as Rosenbusch and his school 

 employ it; that is, it will denote all such rocks as may be con- 

 ceived to be the differentiation-products of a foyaitic magma. 



In the following discussion the terms "uplift" and "depres- 

 sion " will be frequently used. Suess in ' Das Antlitz der Erde' 

 has propounded the doctrine that all movements of the earth's 

 crust are downward. The merits of this view will not be dis- 

 cussed here, although I believe that, in the mechanics of the 

 earth's crust, not only is relative uplift possible, but it has 

 frequently taken place. It must, however, be understood that 

 in my discussion I have frequently used the term "uplift" where 

 Suess would speak of a " negative movement of the sea," and 

 "depression" where Suess would say "positive movement of the 

 sea." The term " transgression " is used in the same sense as 

 Suess would use it. 



General Discussion. — Lyell, the great advocate of uniformity, 

 laid it down as a general rule that in past geological epochs 

 sediments were deposited, lavas were poured out, and intrusions 

 were formed by the same processes which we see going on at the 



