Vol. 54.] IN SOUTH WALES, DEVON, AND CORNWALL. 



267 



About 1| mile south of the confluence of the Tavy with the 

 Tamar, the latter river is crossed by Saltash Bridge. Borings 

 made about 1855, prior to the commencement of the bridge, proved 

 the rock-bottom on the Devon side near high-water mark to be 

 13 feet below the level of low water, beneath 28 feet of silt with 

 slate and stones. The rock was followed for 360 feet into the 

 river to a depth of 30 feet below the level of low- water spring- 

 tides, when it slopes downward at 1 in 5. At low-water mark, 

 where the eastern pier of the bridge stands, there is 18 feet of blue 

 silt and small slate, with large stones occasionally, and farther out 

 in the river the rock-bottom is covered by 3 feet of blue silt with 

 fragments of shells. 



About the site of the central pier the bed of the river was 

 examined by 175 borings made inside a 6-foot cylinder sunk 

 into the mud at 35 different places. Where the central pier 

 stands a cylinder 37 feet in diameter was sunk to the rock, 

 which was ultimately laid dry. The rock-bottom was found to 

 be irregular, but with a general slope of 1 in 6 to the Cornish 

 side, in which direction it was traced to a depth of 76 feet below 

 the level of low-water spring-tides, sloping westward at 2| to 1, 

 with 24 feet of mud and silt over it. Immediately over the rock 

 yellow clay and gravel, gravel and stones, and large stones are 

 recorded. Oyster-shells in considerable numbers were found in the 

 blue mud and silt excavated from the 37- foot cylinder. 



On the Cornish shore the rock-bottom was found by divers 

 to fall steeply towards the middle of the river, and, judging by 

 the slope on that side, the greatest depth to the rock may be 

 more than the 76 feet to which it was traced. The rock at 

 the central pier was a trap or greenstone, so hard that tools could 

 with difficulty be got to work it. A dyke of igneous rock in the 

 clay-slate crosses the river on the line of the bridge, and four others 

 are shown on the Geological Survey map across the mile of narrow 

 channel at Saltash, above and below which, in the claj^-slate, the 

 Tamar widens out considerably. 



Half a mile from Saltash Bridge, on the Cornish side, a timber 

 viaduct crossed a short creek called Coombe Lake (fig. 11). The 

 rock-bottom was proved to 



reach 35 feet below the level 

 of low-water spring-tides, the 

 sides sloping 1 in 2 and 1 in 

 2 1, corresponding with the 

 slopes above water. The via- 

 duct has in recent years been 

 reconstructed in masonry, and 

 the pier-foundations gave the 

 same section of the rock-bottom, 

 but resting on the rock was 

 found a bed of stony clay, about 

 a foot thick. The upper 2 or 

 3 feet of the rock (Devonian 



Fig. 11. 



stony 

 Clay- 



[Scales ; Horizontal, 400 feet=l inch ; 

 vertical, 100 ieet=l inch.] 



