468 



ME. 0. A. DERBY ON 



The larger one (fig. 5 ), which is 9 inches long and 4 inches wide, 

 is a blue phonolite, with tolerably abundant crystalline inclusions in 

 the left-hand portion, which become rarer towards the right. On 

 the left side there is also a long, curved, ribbon-like inclusion of 

 foyaite, which shades off at the lower end into the including rock 

 through a group of scattered crystals, such as are common in phono- 

 lite. The other one (fig. 6), which is about 4 inches long, is repre- 

 sented above as it appears on an irregularly fractured surface. The 

 dark-shaded portion is felsitic, and may, I think, be considered as 

 phonolite, notwithstanding its abundant crystalline inclusions. It 

 forms a distinct sheath, sharply defined against the enclosing foyaite, 

 about the whole inclusion, and also about the three principal 

 crystalline masses imbedded in it in the lower part of the mass. 

 It appears, however, to shade into these, and into the smaller and 

 less-defined inclusions of the upper part. These masses differ con- 

 siderably in aspect from the enclosing foyaite, and are flecked with 

 small dark patches apparently related to the felsitic mass. As 

 the specimen has been placed in the hands of Prof. Rosenbusch, no 

 further description of it will be attempted here, as, for my present 

 purpose, it is sufficient to signalize the double nature of the inclusion, 

 that is to say, of a rock of granitic texture in one of felsitic character, 

 which is itself enclosed in a rock of granitic type. The appearance 

 of this and other inclusions, coupled with the facts already stated, 

 as to the occurrence of phonolite as a peripheral facies of foyaite, lead 

 me to regard the inclusions, whether in the one or the other of the 

 rocks, as parts of the same original magma. A petrographical exam- 

 ination will doubtless determine whether this view is correct or not. 



Prom the cutting in foyaite, above mentioned, to the Cascata 

 station the road winds for about a kilometre around a prominent 

 spur, some 400 metres wide, of bluish-black and greenish tuff, which. 



