Notes on the Igneous Rocks. 467 



Rocks and Effusive (Volcanic) Rocks. The structures char- 

 acteristic of each of these groups is stated. They are then 

 classified in eight vertical columns, according to their 

 mineralogical and chemical composition, into rocks having 

 as an essential constituent an Alkali Feldspar ; an Alkali 

 Feldspar with Nepheline oi- Leucite ; Nepheline Rocks ; 

 Leucite Rocks ; Melilite Rocks ; Rocks composed essentially 

 of Nepheline or. Leucite and Plagioclase ; Plagioclase (or 

 Soda Lime Feldspar) Rocks ; and lastly, Rocks containing 

 no Feldspathic Constituent. 



These rocks are then subdivided according to the bisili- 

 cates and micas which they contain, while further sub- 

 divisions are made in the case of the more acid rocks by 

 the presence or absence of quartz, and in the case of the 

 basic rocks by the presence or absence of olivine. 



The part of Prof. Rosenbusch's scheme of classification 

 which has met with the most adverse criticism is the 

 group of the Dyke Rocks. It seems rational to suppose, 

 however, that since in an extinct volcano we have a crack, 

 pipe or dyke, at the lower end of which we have typical 

 Plutonic rock, and at the upper end typical Volcanic rock, 

 and since these two rocks differ widely in structure, that 

 we might have in the intervening position a rock or rocks 

 with a peculiar structure of their own. Prof. Rosenbusch 

 believes, after the study of a great series of dykes, that 

 these rocks have certain distinct structures of their own, 

 and that although in some cases rocks exhibiting these 

 structures occur as facies of Abyssal or Effusive rocks, as, 

 for instance, about the borders of Plutonic masses, yet as 

 independent geological units rocks possessing these struc- 

 tures never occur except in the form of d} T kes. These rocks 

 which he believes cannot properly be referred to either of 

 the other classes, he has placed together by themselves as 

 Dyke Rocks. In some cases, as in the Aplites and the 

 Minettes, they have pronounced and easily recognized char- 

 acters, in others the characters approximate more nearly 

 to those of the Plutonic or Volcanic rocks. Following 

 Prof. Rosenbuch, these Dyke Rocks have been divided into 



