T. Davidson ^ G. Maio — U. Silurian Rocks of Shropshire. 101 



ultimately be the subject of the laborious process of " hand-picking," 

 it is desirable to reduce its bulk as much as possible. The whole is 

 first passed through the sieve of one-inch mesh, which catches all 

 the stones, lumps of undigested shale and the larger fossils, which 

 are easily picked out. The mass is thus reduced to from half to 

 two-thirds of its weight, and is then dried. It greatly facilitates 

 further operations, to sort this into separate sizes by passing 

 the dried material successively through the sieves of ^ inch, ^ of 

 an inch, and -^V of an inch mesh. The fine matter passed through 

 the xo of an inch mesh seldom contains fossils, and may be thrown 

 away. 



Now comes the final process of hand-picking from the three sorted 

 lots of debris. These are spread out thinly on a slab of slate or a 

 smooth board, and women, at a wage of Is. 6(i. a day, quickly per- 

 form the operation, and readily learn not only to pick out the fossils 

 from the gravelly debris, but also to roughly sort the sijecies. 



As an instance of the good results of this process, we would men- 

 tion that from one cartload of the Buildwas Beds of Wenlock Shale 

 no less than 4300 specimens of one species, Orthis biloba, were 

 obtained, besides a much greater bulk of other Brachiopods, amount- 

 ing together to 10,000 specimens at least ; but this does not nearly 

 represent the full wealth of life of this rich horizon, as many of the 

 larger species, and others not completely calcified, would get broken 

 up in the washing process, and we have had to supplement the 

 species obtained by washing with a series of hand-picked specimens. 



The whole of the debris has been preserved after picking out the 

 Brachiopods, as it abounds in minute corals and other fossils, which 

 will we hope be investigated by other observers. 



The cost of the process, with the aid of a potter's claj'' blunging 

 machine, amounts to about 18s. per ton of materials. This includes 

 the cartage of the shale two or three miles, the whole process of 

 washing, and the hand-picking of the fossils by paid workers. 



The following estimate of the thicknesses of the several sub- 

 divisions of the Upper Silurian rocks of Shropshire is based on the 

 average of three sections from S.S.E. to N.N.W. across the north- 

 eastern end of the great Shropshire escarpment. One of these passes 

 through the town of Much Wenlock, and the others at distances of 

 about two miles to the east and west. 



The horizontal distances of the lines of contour from the base of 

 the Upper Llandovery to the base of the Devonian average from 3^ 

 to 4 miles or about 20,000 feet ; and taking the general dip at 12°, the 

 total thickness of the Upper Silurian series can scarcely be less than 

 4600 feet. 



An estimate of the actual thickness of each of its subdivisions is 

 difficult to arrive at accurately, mainly from the fact that most of the 

 zones, both in mineral character and in the range of species, 

 insensibly graduate into each other, and it is probable that no two 

 observers would fix exactly on the same lines of demarcation. 



There are perhaps few parts of the country in which the surface 

 features of contours are ruled so closely by their geological structure 



