THE ARCH^AX FORMATION OF THE ABUKUMA PLATEAU. 277 



mained since without any marked terrestrial disturbance. The black 

 dykes are surely of post-archagan age. 



Deferring details about the diorite-porphyrite to subsequent 

 pages, I shall first speak of the material of the pegmatophyritic chjkes. 

 It is flesh-red and compact, breaking into polygonal blocks with 

 sharp edges. Under the microscope it is very simple, as in its macro- 

 scopic aspect, but it shows a very instructive structure. The main 

 mass is made up of nothing but quartz and orthoclase, regularly 

 intergrown and showing a hieroglyphic or à la grec form. The 

 only other ingredient, that is visible under the microscope, is a 

 skeleton-like magnetite which is transformed at the margins into 

 brown sesquioxide to which is largely due the reddish tint of the 

 rock. The quartz and feldspar are in about equal quantities ; con- 

 sequently it is difficult t(3 decide which of them should be considered 

 as the main mass. The latter is for the greater part kaolinized and 

 dirty, while the fresh quartz is charged with swarms of liquid- 

 inclusions. 



The pegmatophyritic dykes occur, as has been already stated, in 

 regular bands of uniform but moderate breadth, traceable for con- 

 siderable distances. Apophyses have so far not been observed. 

 Such being the condition in which the pegmatophyre occurs, I felt 

 considerable diffidence in deciding as to its origin, whether, that is, 

 it is a primary secretion after the manner of a miarolitic mass, or a 

 filling up of cefts by lateral secretion, or lastly, an injected magma 

 from the earth's interior, solidified in the present form. The last 

 supposition seems to be the most probable, as the rock traverses 

 indiiferently all the varieties of granite, and even those of amphibo- 

 lite. 



The macroscopic development of the preceding is, to speak morphologically, 

 nothing bat pegmatite dykes, of which we have opportunities to meet ivith in 



