460 
JOURNAL OF THE PLYMOUTH INSTITUTION. 
is broken at several points. The Tamar, after rounding its western 
extremity, passes through it at Cremill; the Plym has a narrow 
channel at Cattedown ; and at Stonehouse Pool, Millbay, and 
Sutton Pool, the waters of the Sound find access through the 
barrier to basins worn out of the slate rocks behind. 
The limestone varies much in colour, structure, and dip. Its 
most constant features are its crystalline character, and the 
regularity of its divisional planes. Both on the north and south 
it graduates into the slate through calcareous shale. Bedding is 
frequently indistinct, and in some central parts of the mass 
apparently non-existent. It abounds in fossils — chiefly coralline in 
its more massive portions ; whilst some of the exterior beds have 
yielded large quantities of bivalves and univalves ; and others, with 
the adjoining slates, are remarkably fruitful in crinoidal remains. 
In texture it is generally highly crystalline, and in colour very 
various, ranging from black, through red, yellow, brown, dove and 
gray, to white. There is a marked increase of dip from north to 
south. Whilst on the northern edge the dip varies from 20° to 40°; 
on the southern it runs from 60° to 75°. At Cattedown there is a 
shallow synclinal and some undulation, but the prevailing dip is 
towards the south. 
Both the joints and the crystalline structure of the limestone are 
due to causes subsequent to the formation of the reef. To sub- 
sequent causes also must we attribute the fact that portions are 
dolomitic, containing a large quantity of magnesia, which has been 
exchanged for the lime. In the Yealmpton limestone dolomite 
abounds. 
And now to return to our reef. At length the building came to 
an end. There may have been a descent of the ocean bed too 
rapid for the builders to keep pace with. They may have been 
overwhelmed by some great discharges of detrital matter ; they 
may have perished in a fresh outbreak of volcanic forces, though 
recent volcanic action does not in all cases absolutely prevent the 
growth of coral. But they did not disappear finally without a 
struggle for existence. 
And then we enter the third division of our local system. Its 
characteristic is sandstone, but this does not immediately appear. 
The rocks of this group are best studied, though they continue 
across the Sound, on its eastern side, from Mount Batten south- 
ward. They present an interesting but most complicated study. 
