530 



DE. C. CALLAWAY OX THE GENESIS OF THE 



a folium, passes across a transverse crack which therefore must 

 be older than the folium, and presumably older than the general 

 foliation in its completed state. Where a transverse crack passes 

 through quartz, it sometimes vanishes for a little distance, 

 reappearing further on, so that it is clear that the sides of the crack 

 have come together and become welded. The occurrence of these 

 transverse cracks, and their evident connexion with the process of 

 metamorphism, is a piece of evidence which tends strongly to 

 confirm the suggestions made by the structure of the folia. 



Mica-schist, formed from Felsite. — A very interesting section 

 (fig. 2), about 30 feet in length, is seen in the slope at the end of the 



Fig. 2. — Passage of Felsite into Schist. 



1. Crushed Felsite. 2. Schistose Felsite. 3. Schists. 



south-eastern spur of the Eaggedstone Hill. Taken in descending 

 order, the following are the rocks observed : — 



1. Pale-reddish felsite, so jointed and crushed that it was difficult 

 to obtain a piece large enough for a microscopic slide, and the 

 specimen selected broke into fragments in grinding. This rock 

 forms a band, striking across the ridge in concordance with the 

 foliation of the underlying schists. Under the microscope the rock 

 is seen to be entirely devitrified, and the microcrystalline structure 

 of a typical felsite is very apparent. Even minute fragments, 

 which have remained entire in grinding, are brecciated. The cracks 

 are usually marked by iron- oxide, and, occasionally, when wider 

 than usual, are filled by quartz or by mica. There is a rough 

 parallelism in the structure, and this incipient foliation is some- 

 times accentuated by a little mica. 



2. A few feet below the last, and separated from it by soil. 

 The rock is still felsite, much sounder under the hammer than 

 Ino. 1. llicroscojDically examined, the field is seen to be clearer, 

 the parallel structure is more distinct, and there is a larger pro- 

 portion of mica. This rock has evidently been porphyritic, for 

 there are several deformed masses of quartz on the slide. One of 

 these is ej-e-shaped, and tails out at each end in a stream of minute 

 granules. 



