*r 





w 





k 



% 



U 



\m\ } 







ki 



he 





II * 









VOLCANIC ROCKS OF DEVON. 



231. — Schistose vesicular volcanic ash. 

 vesicles are filled with calc spar. — Tavistock. 



251 



Some of the 



Fppee 



Gaileet. 



232.— Schistose vesicular volcanic ash, showing th* ^all-cases 



, ,. . . _ _ > ,,lu h tuo 6 and 7. 



bedding, joints, and cleavage planes— Tavistock. 

 233. — Very vesicular volcanic cinder. 



234. 



( 



filled with calc spar), with more solid portions, probably of 

 contemporaneous lavas, upon the whole resembling a sub- 

 stance composed of finely comminuted volcanic matter con- 

 solidated — Brent Tor. 



Scoriaceous lava, volcanic cinder. Brent Tor. 



235. 



TOADSTONE AND AMYGDALOID. 



236— Toadstone,* containing pebbles of limestone. 

 Jbrom the deer ' " " 



Derbyshire. 



High 



near Tideswell, 



A 





J 



' 







The name loadstone is applied to the greenstone which 

 ..curs in interstratified with) the Carboniferous limestone 

 ot Derbyshire and the north of England. 



The term is derived from the German todtstein (dead- 

 stone), denoting the absence of minerals in the beds with 

 which it is associated— H. W. Bristow. 



Toadstone is a volcanic rock or lava truly interbedded in 

 the Carboniferous limestone of Derbyshire, and is probably 

 oi the same geological age with the volcanic rocks of the 

 lower Coal measures of Scotland. 



It frequently assumes a cellular structure. Sometimes 

 the cells are empty, while at others they are filled with 

 calc spar, green earth, and other minerals, and in the latter 

 lorm they a re termed amygdaloids. The vesicles have 



lav!!T % IT f ° rmed bj thG GSCa P e 0f S ases ' as in ^dern 

 hit (See ^°f; 4 and 5 ' Case 2 )' and the kernels with which 



of If? ir ^ bGen f ° rmed b ^ the S radual ^filtration 

 dissolved lime or other matters into the cavities. Fre- 



* 



* See also Nos. 188, 190, 191, and 253. 



