Maryland Geological Survey 11? 



the suspension. This appears to have been more time than Thoulet 

 allowed, but it was probably on this account that very little residue from 

 the material tapped from the separating funnel was obtained. Another 

 slight adaptation of Thoulet's method consisted in fixing the time for the 

 last settling at thirty seconds. That is, all the " sand " and " silt " had to 

 settle in that time. This period was chosen on the basis of practical 

 experience with the samples, which showed that the interval was sufficient 

 to allow all but a certain cloudiness to settle out. In many samples, how- 

 ever, it was found that there was a sort of transition material which not 

 only had a different appearance from the sand but also did not seem to 

 settle with the same promptness as the sand, forming a sort of interme- 

 diate constituent. Microscopic examination justified this conclusion as, 

 according to the constitution of the sample, irregular glauconitic frag- 

 ments, limonitic fragments, or small clay flakes secondarily cemented 

 appeared in this intermediate product. The determination of the amount 

 of this product settling in thirty seconds but not in ten, was most 

 unsatisfactory, since the quantity depended largely on the amount of 

 water in the evaporating dish, the temperature of the water as affecting 

 convection currents, and probably other factors, so that it was possible 

 to wash back much of the material that had once been washed out, and 

 vice versa, by continuing washing to keep on almost indefinitely washing 

 out a little more silt from the sand. Any absolute value, of course, the 

 portion settling between thirty and ten seconds has not in any ease, since 

 it represents no distinct pure product of any kind; but even its relative 

 amount in different samples has no great precision. Actual results, how- 

 ever, as given in the following analyses, show that the differences in 

 quantity are marked enough in some cases to indicate roughly the amount 

 of this product, and thus to give some indication of the extent of the 

 processes — in most cases probably subsequent to the formation of the 

 sediment — which have produced it. Besides, since the material is finer 

 grained than the extra fine sand, it is, in the end, according to Thoulet's 

 classification, counted with the clay to determine the amount of mud, so 

 that the separation of it does not affect the final numerical result. 



