CHAPTER V. 



THE OCCURRENCE AND SUCCESSION OF ROCKS. 



Methods of determining succession. — Determinations of the order of succession of 

 eruptive rocks involve considerable difficulties. Superiuiposition alone is 

 an insufficient indication of relative age, for intrusions and laccolitic accu- 

 mulations of younger rocks may underlie older ones. Neither are inclu- 

 sions of one rock in another always a safe guide. Cases are not unknown 

 where intrusive masses of a younger rock in an older might readily be mis- 

 taken for inclusions of an older rock in a younger one. I have even ob- 

 served instances, though not in the Washoe District, of slabs of older 

 rocks embedded in later eruptions in such a manner that but for other and 

 overwhelming evidence as to the order of succession, they might have been 

 interpreted as dikes of the older rock in the younger. Moreover, when the 

 rocks in question are closely allied, as is very frequentU' the case, local 

 modifications of one rock may readily be confounded with inclusions of a 

 different but similar species. Such an error is peculiarly likely to occur 

 where there is brecciation. As has been pointed out on page 82, masses of a 

 single rock subjected to partial decomposition may also simulate inclusions 

 or dikes of one rock in another. Thus while at first sight it might appear 

 that dikes and inclusions furnish the most unimpeachable evidence of suc- 

 cession, this class of evidence is peculiarly deceptive except where the rocks 

 are fresh and characteristic, the exposure perfect, and the cases abundant 

 Where any of the rocks are very recent, evidences of erosion form an im- 

 portant argument as to succession, as will be seen from the remarks on the 

 later hornblende-andesite. 



No single method of determining the succession of eruptive rocks is 

 ordinarily sufficient, and due weight must be given to all the facts bearing 



