TRANSACTIONS OP THE SECTIONS. 93 



suvftice, excepting at the few points of contact, than the sugar on candied citron ; 

 and under this crust the glaze-wash had, as the heat rose, melted into a complete 

 glass. 



What could this incrustation be but a small part of the mineral matter whicli had 

 been so largely dissolved away below, by steam at an intense heat (aided no doubt 

 by the alkali in the wood fuel), as to have much exceeded two hundredweight, and 

 which left trac^es of its flight in a sprinkling of hoar-frost, precipitated at a red heat ; 

 the glaze happily intervening to prove that it had a source altogether e.xtrinsic 

 from the ware on which it had lighted ? 



An Estimate of the quantity of Sedimentmvj Deposit in the Onmj. 

 By the E,er. J. D. L.v Totjche. 



The Onny is a .small stream in the south-west of Shropshire, the waters of 

 which are collected over an area of 81 square miles. It is well known to geologists 

 as flowing through the principal strata of the Silurian rocks. 



The following means have been taken to ascertain the rate of denudation of 

 this valley. 



At the commencement of the present year a gauge-post was erected at a con- 

 venient spot in the river, at Stokesay, and a register has been kept of the height 

 of the flood by means of it. 



A number of experiments have been made to determine the maximum surface- 

 velocity of the stream, and from these a Table (A) has been constructed, which at 

 a glance gives approximately the mean rate for every decimal of a foot, and the 

 amount of discharge per minute in cubic feet. For 'this purpose Table VII. in 

 Neville's ' Hydraulics ' was used. 



At the commencement of each flood, at its full height and decline, specimens of 

 water were collected in ordinary quart bottles, which contain about 2G ozs. each, 

 and the sediment after subsiding w<is filtered and accurately weighed, the filter- 

 paper being first carefully dried. Table B is an extract from the register of these 

 experiments. The entries only include the more important floods. 



The headings of the columns will speak for themselves ; with reference to the 

 fourth column, it has been found that the number of grains in the 100 ozs. of 

 water varies considerably with the place from wliich it is taken. That collected 

 close under the edge of a weir was 20-o per cent, higher than that collected about 

 thirty yards lower down. The difference is evidently due to the greater disturb- 

 ance of the water therc^ than elsewhere. 



I think of constructing a filter on a large scale, with wire gauze or some such 

 material, to be suspended for a certain time under the weir during a flood, to arrest 

 the coarser sand and pebbles, which cannot be estimated by the foregoing process. 



The fifth column gives the number of pounds of dried sediment which pass down 

 the stream per minute. 



To give an idea of what may be done with these data, I have made the follow- 

 ing calculation. The specific gi-avity of dry Silurian rock is about 2-5. Suppo- 

 sing the highest rainfall and the highest observed percentage of deposit to take 

 place for six weeks in each year (and this seems to be a very extreme estimate), it 

 would take 430 years to wear away a single inch of the surface of the valley of the 

 Onny. Of course this is far from being an accurate estimate, but it is evident 

 that it would be possible to approach indefinitely near to accuracy by these me.ans, 

 and therefore to a true estimate of the rate of denudation of the Tand. 



The last column gives the average rainfall of four' rain-gauges placed at different 

 points of the basin of the Onny. 



[t will be seen that, as might be expected, a certain relation exists between the 

 sedimentary deposit and the rainfall, though many things tend to interfere. The 

 suddenness of rain, its occurrence after a drought, or after long-continued wet 

 weather, will do so. But there is reason to expect that extended observation will 

 establish a general correlation between the average rainfall and the average rate 

 of denudation. 



