79 



taken the two valves of .. Ic-shell, place i 



in their original position, and, after making a small hole 



in one of them, pours in liquid j>last< it until the 



ial cavity is filled with it. he plaster has 



< an ri-mM\r the \alves, and he will have an internal 

 cast of the cockle. But had he seen the object before 

 making the exprnnu-m, he would not have been lik 

 guess what it was < 



Again, at the outset he may experience some diffu ulty 

 the same fossil when it occurs in different 

 \ample, a when pre- 



served in shale or any argillaceous layer, may retain each 

 leaflet, scar, and surface-marking, will perhaps appear in 

 sandstone as a mere black streak of coaly substance. A 

 fossil fish, whi< h it ! t limestone nodule may have 



scale and bone in place, each with its peculiar 

 M ulp 1 itely shown, may, it met with in a conglo- 



i Merely in scattered fragments, all so much 

 rounded and worn as to tx : ccognisable. 



A little cxper 1 guide the learner to those rocks 



i are likely to contain fossils. No general rule can 

 be laid down ; for the kinds of rock which arc barren of 

 organic remains in some places, abound with them in 

 others. Conglomerates, for example, are not usually rocks 

 in which we should expect to meet with fossils ; nor as a 

 rule do we find them there. Vet there are many richly 

 fossiliferous conglomerates, such as those of the Silurian 

 rocks of Penwhapple Glen in Ayrshire, and of the Upper 

 Old Red Sandstone in several parts of Scotland. Ar- 



ous rocks are commonly better grounds for fossil- 

 hunting than sandstones, and limestones are better than 



