TIDAL AND GENERAL NOTES ON CATTEWATER. 421 
September 6th, 1833, he found the distances to have decreased 4 
feet at the north-westernmost stone, 3 feet 6 inches at the middle 
stone, and at the southernmost stone 9 feet 6 inches — the measured 
distances being respectively 35 feet, 34 feet, and 17 feet. "I 
observed also," he adds, " that in the course of these years nearly 
as much of the sea wall had been washed away as was left standing. 
In the course of time it will all go, and the low land be left to the 
mercy of the sea, the water at length make a breach over it, and 
eventually it may become similar to the bridge between Nicholas 
Island and Mount Edgcumbe." 
Mr. Blackburn's prophecy has not been fulfilled, nor does it 
now seem likely to be. The completion of the Breakwater a few 
years later than his last observation has apparently removed all 
danger from that side; and it is not at all unlikely that the 
greater part of the waste he records had taken place in the earlier 
years of the period, before the Breakwater had begun to afford 
protection. The waste since 1833 cannot at any rate have been 
very serious, though its exact amount is not definitely ascertain- 
able, and there is only a small fragment of the wall now (1887) 
left. Such encroachments as have taken place of late on the 
seaward face of the isthmus appear to be of a local and casual 
character. On the other hand there is ample evidence that a good 
deal of the low earthen cliff on the Cattewater side has been 
gradually wasted by the tide undermining the bank, and that this 
process is still in operation. 
The bulk of Mr. Blackburne's MS. is occupied with a descrip- 
tion of the Turnchapel Dock and the means adopted to make it 
watertight and available for occupation, the original arrangement 
being most defective. Probably the most practically interesting 
part in a scientific point of view is the description given of the 
rock in which it was excavated. From the head, half way toward 
the dock gates, it was cut out of schistose rock in which were 
numerous fissures pervious to water, containing loose soil and 
decayed slate. Further north, on the east in particular, there was 
loose slate. The ground under the apron was a sort of clay mixed 
with small pieces of decayed slate, and there was a good depth of 
it, excepting at and near the western pier, where the slate rock 
approached. The schistose rock was bounded both north and 
south by limestone. At about 60 feet from the head of the dock 
southward it joined the limestone, and at about 200 feet from the 
