our. xii 





16; 



the mass, but with the component mineral! entirely 



It would seem that, ai a rule, the extent of alteration 

 in the rocks adjoining an intrusive igneous maai bear* 



some proportion to the size of the latter. We may be 

 prepared for traces of the change at a greater di 

 from a large injected sheet than from a small d 



The " Necks " which mark the sites of former volcanic 

 funnels commonly form rounded or conical hills (Fig. 

 59). They consist sometimes of crystalline rocks, 

 sometimes of tuft or coarse agglomerate. These materials 



FM. 6a-<Jroad.pkn oTib* ractm of tb. N* thown b Pi* *. 



descend, of course, vertically through the surrounding 

 rocks, \\hn\\ are sometimes considerably altered all 

 round (Fig. 60). Necks vary in size from a few yards to 

 a mile or more in diameter. Many interesting examples 



