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REV. E. HILL ON THE ROCKS OF GUERNSEY. 409 
Fig. 1.—Grooved Weathering in Dioritic Rock, a Quarter of a Mile 
south of Fort Doyle. 
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or 10 feet thick, in the midst of coarsely crystalline rock, dipping 
about 50° or 60° to the N.E., visible on both sides of the headland 
and running into the sea. I thought at first that they passed con- 
tinuously both above and below into the neighbouring crystalline 
rock, and that if they were sedimentary, so must it be also. But 
doubts expressed by Professor Bonney after examining slides from 
the rocks led me to make a more minute inspection than before. I 
had already noticed that the banded appearances were narrower and 
less conspicuous on the western side of the point. Investigating 
this, it appeared that some striping existed in the crystalline rock 
itself. 1 then saw that in places weathering developed this striping 
till it bore a close resemblance to bedded structure. Finally I was 
able, though with much difficulty, to trace an actual line of division 
between the slaty beds and the surrounding crystalline mass. 
With hesitation, then, I venture to suggest the following explana- 
tion for this place. The igneous rock has caught up or surrounded 
a large slab of slaty beds, and structures have been set up in the 
igneous mass parallel to the surface of the slate. These structures 
weather into mock bedding, and cause the appearance of a continuous 
passage from normal igneous into normal stratified rock. It is not 
easy to be certain about the exact nature of these structures. They 
may perhaps be surfaces of crushing, where the rock has yielded 
along certain planes, such as Prof. Bonney has described in the 
‘Geological Magazine,’ 1883, p. 486. Several yards to the south of 
the slaty beds is a thin streak not an inch wide, extending for many 
feet, which can scarcely be anything else (a section cut from it is 
described by Prof. Bonney). It seems to follow a joint, and has a 
horny texture, which runs in wavy lines through the crystalline 
-syenite-like rock. Other similar streaks I have noticed along the 
northern shore. 
It is obvious from the shattered condition of the rocks of this 
island that they have been subjected to enormous pressures. Where 
a rock under such pressures gives way along a plane, and one surface 
slides over the other, the powder which the rock will be ground 
into would probably resemble sedimentary material. Heat would 
be developed ; would this not assist in recementing it ? 
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