itETEOElTES FOUND IN SEAMS OP COAL. 311 



THE RHYDGALED STONE. 



This, as previously mentioned, was found in November, 

 1850, in the main seam of coal at Rhydgaled, near Mold. 

 It is composed of a crystalline stone of a grayish-white, with 

 some small streaks and spots of a black colour dispersed 

 throughout the mass. The outside is generally smooth, but 

 contains a few little holes on its surface. It is coated with 

 a shining black polish, just like a thin varnish, without pene- 

 trating far into the body of the stone, as in the two last 

 specimens. There are marks of slickenside on the outside, 

 but not so strong as those on the specimen from Patricroft, 



Fig. 3. 



Its form (see fig. 3) is that of 

 #''^^ an irregular oval, with three of its 



sides and one end compressed. 

 The greatest length is 5^ inches 

 V by 2 1 inches in breadth. It 



weighs 1| lb. avoirdupois, and 

 has a specific gravity of 2r60." Dr. Smith's analysis of it is 



as follows : — 



Silica 99-182 



Alumina "649 



Protoxide of Iron '022 



Lime *016 



99-869 

 All the three stones were found in seams belonging to 

 the middle division* of the coal field, the two first named 

 in the higher, but the last named in the lower portion of it. 

 Had they been found in the rough gritstones of the lower coal 

 field, where most of the sandstone rocks prove that consider- 

 able currents of water had been in action ; and that even 

 some of the seams of coal, especially one known by the name 

 of the Feather Edge Coal in Lancashire, are sometimes 



* For the definition of this part of the coal field, see Transactions of 

 the British Association for the Advancement of Science, toI. xii. p. 46 ; 

 and Sturgeon's Annals of Philosophical Discovery, and Monthly Reporter 

 of the Progress of Practical Science, vol, i. p. 126. 



