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breadth of many yards down to the merest thread. A 

 marked variation in lithologiral character may be ob- 

 served as a rule to attend these changes in the dime n 

 sions of the veins. The larger veins may consist of 

 talline rock, like that of the main mass ; but as they 

 ramify and diminish they pass into an clvanite (micro- 

 granite) or fine-grained felsite. 



2. Interstratifled igneous rocks are sheets of vol- 

 canic material which have been 

 ejected at the surface and have 

 solidified there. Consequently, 

 whenever they can be satis- 

 factorily determined, they show 

 the former existence of volcanic 

 activity on or near to their site. 

 The geological date of their 

 eruption can be ascertained by 

 an examination of the strata 

 with which they have been 

 contemporaneously associated. 

 Thus, for instance, if we find a 

 set of lavas and tuffs inter- 

 stratified with limestones and 

 shales containing Productus 



RNUiNfttimfUt 



lavas; t, t, tuff.; i, limestone with A thy r is ambigua, Lithostrotion 



marine organisms (Carboniferous . 



Limestone); 2, shale ; 3, sandstone JUnceum, etc., WC Conclude With 



4, fireclay; 5 , coaL out hesitation that as these are 



unequivocally Carboniferous Limestone fossils, the vol- 

 canic eruptions must have taken place during the period 

 of the Carboniferous Limestone (Fig. 64). 



