14 Hoivard Fox — On a Soda Felspar Rock — 



irregular, and sometimes of sucli forms as would be yielded by 

 rhombs ; they are undoubtedly pseudomorphs after a ferriferous 

 carbonate. It has, in places, all the appearance of a stratified rock, 

 and is distinctly banded. 



In one locality it assumes a nodular form with the nodules at the 

 outer edges of the rock, showing a radiated or spherulitic structure. 

 The spherulites vary from 2 to 10 mm, in diameter. 



The greenstone is posterior to and intrusive in this rock, and 

 abruptly cuts across, bends, and disorders its beds. Extreme 

 crushing has, in places, altered the original junction-line between 

 tbe two rocks, and thrust-planes and fault breccias are seen. Quartz- 

 veins traverse both rocks, but are more numerous in the soda rock. 



The slate is occasionally full of ferruginous spots, and contains 

 rusty-brown concretions in the same manner as the soda rock ; it is, 

 on the northern side of the headland, interbedded with bands of blue 

 limestone from a few inches to several feet in thickness. There 

 are a few bands of dark impure chert at the western end of '^Q 

 headland. 



A rock, rich in soda, called Adinole is said to occur as a contact 

 product due to greenstone in the Hartz.^ The question arises 

 whether this Dinas Head rock is an Adinole or a soda felsite. 

 Before stating the difficulties in the way of either hypothesis, we 

 will examine the headland more in detail. 



In approaching Dinas Head from Trevose Head there are blocks 

 of quartz lying about on the gi'ass which from a distance look like 

 sheep. These blocks are apparently fissure quartz. One of them 

 which lies nearest to the public road to the lighthouse is composed 

 of orbicular quartz, internally radiated, the individual radiated 

 spheres varying in size up to 1 inch in diameter. 



The isthmus connecting Trevose Head with Dinas Head is bare of 

 soil, the slate exposed is thinly laminated, bluish-brown, and friable, 

 with traces of badly-preserved organic remains. The slates show 

 reeding structure. The sea-water that breaks over it from the north, 

 has worn channels to the edge of the precipitous clifi^ on the south, 

 side at "a" in the annexed Map (which is copied from the 25-inch 

 parish map, and may be taken to be approximately accurate, though 

 only indicating some of the numerous faults). In one of these 

 channels, alternate beds of this soda rock and of slate are exposed 

 for a length of tliirty feet, dipping in a northerly direction in con- 

 formity with the prevailing dip of the district. The beds of the 

 soda rock vary from 1 inch to several feet in thickness, they weather 

 bluish-white, with prominent thin parallel bands or ridges so closely 

 set that there is often only 1 mm. between them. The surface is 

 pitted all over with minute cavities and brown spots. Mr. Teall 

 kindly examined a section of this rock, and sent me the following 

 description : — 



" Compact banded bluish-gray rock with brown rusty spots. 

 Under the microscope the structure of the main mass of the rock is 

 seen to be crypto-crystalline, but the nature of the material cannot 

 1 Vide Brit. Petrology, Teall, pp. 217-219. 



