418 



GEOLOGY, 



other minerals being regarded as accessories. Magnetite or ilmenite is 

 almost universally present. The varieties are usually designated by 

 prefixes, as olivine-dolerite, enstatite-dolerite, etc., but special names 

 are also used for some of these. 



Fig. 346. — Conglomerate, Carboniferous series. Bancroft Place, Newport, R. I. 

 (Walcott, U. S. Geol. Surv.) 



The ancient dolerites have usually undergone internal changes and 

 such rocks are often called diabases. While the use of the term has not 

 been uniform, it accords with the better practice to regard the diabases 

 simply as partially altered dolerites and basalts. In general, therefore, 

 the diabases are but ancient dolerites. 



General names. 



The difficulty of distinguishing many of the foregoing rocks from 

 each other by any means available in the field, owing to the minuteness 

 of the crystals, and to the gradation of one type of rock into another, 

 makes it desirable to employ certain general names which will correctly 



