1880.] ATL [ Wadsworth. 
whose origin is known; and when we find the same characters in 
other rocks of unknown origin, we are enabled from these marks to 
determine the origin of the rock of unknown history. Eruptive, 
chemical, and mechanical deposits are being formed ‘to-day on the 
earth’s surface. Their origin is known so far as it relates to their 
present position. Other formations have their origin known from 
historical record. The various known cases give the data for work- 
ing back into the past history of the rocky crust of the globe. In 
this study it would seem that the characters and relations of a form- 
ation must prove its origin; and it is not allowable to assume because 
some material of a certain kind is being deposited in one way at the 
present day that al/ material of that kind must have been laid down 
in like manner, unless its characters and relations are the same. It is 
not proper for us to decide a priori the origin of any formation, until 
its conditions have been studied. Furthermore it is not allowable to 
take characters common to formations of unlike origin as proving the 
rock in question to belong to one instead of the other. We must 
choose as deciding points those features that are exclusively, so far as 
known, confined to rocks of one origin. If the diagnostic features 
are common to rocks originating in.two or three dissimilar ways, their 
discovery in the rock in question only shows that it may have been 
formed in any of the two or three ways, but does not show which 
one. 
Doubtless some one at. this point, if not before, exclaims : “What 
nonsense! doés not every geologist and petrographer know these 
things? Why should a paper open with such trite and commonplace 
remarks?’’ We beg the pardon of our critic, and urge in excuse the 
fact that these simple and obvious rules have been repeatedly vio- 
lated in the study of the questions before us. Furthermore the 
history of geology is fraught with illustrations of the neglect of these 
and other, perhaps simpler, rules. In applying the principles above 
given in studying the question before us, it is necessary to study 
the rocks im situ and accept the evidence they there present. 
Taking up the problem of the origin of the iron ore and its associ- 
ated jaspilite it is proper to remark, in order to save time, that 
except in some few secondary and subordinate cases, they do not, so 
far as we have observed, present the characters of vein-stones. All 
writers, so far as we are aware, agree upon this point, and the evi- 
dence has been given in their writings; hence it is not necessary to 
discuss the question here. 
