88 Bulletin of Laboratories of Denison University [Voi. xi. 
the central peaks, forming a monocline with a dip of about 
forty-five degrees in a direction twenty or thirty degrees south 
of west, exposing the carboniferous limestones on the western 
slope and the underlying metamorphic granite and quartzite on 
the more abrupt and irregular eastern slope. Toward the north- 
ern end the limestone has been removed by the effect of a sec- 
ondary eruptive which has served to dislocate and fault the 
limestone series very extensively and to thrust it westward ob- 
liquely across the range at a point north of the Graphic prop- 
erty. 
North of the Magdalena range the effect of the same oro- 
graphic line of weakness or fault which made the range proper 
was felt at a later period and formed the axis of an extensive 
series of trachyte eruptives. The central portion of the range 
was the theatre of enormous eruptive activity during the earli- 
est or andesite period. Two craters of this age remain, known 
respectively as Big and Little Baldy, the first situated at the 
junction of the southern with the middle third, and the sec- 
ond at the junction of the northern with the middle third of the 
length of the range. The northern crater, or Little Baldy, is 
still quite well preserved and the walls of the crater proper ex- 
pose successive flows of andesite, andesite tuff, and aphanitic 
phases of the same material. From this crater a most exten- 
sive rift seems to have been formed to the northward along the 
western faulted margin of the stratified rocks of the range, from 
which fissure, as well as from the crater itself, enormous masses 
of basic lava flowed to the north and west. Other dykes and 
flows in different directions intersect the region. It may be 
said in a general way that, as a rule, wherever these dykes pen- 
etrate or influence the limestone series there is a tendency to 
segregate copper, silver and lead, while in other instances, 
where the contact is with acid rocks, the selveges tend to carry 
gold. It is rare to find gold not directly or indirectly associated 
with these basic eruptives. 
South of Little Baldy and connecting with Big Baldy is a 
series of andesite dykes and bosses, a result of which the lime- 
stone has been displaced eastward into the Water Canon region 
