296 The American Geologist. November, 1903. 
town of McMillen was in a flourishing condition, there was a 
move on foot to utilize the salt of these springs: clams, ditches 
and evaporating' pans were put in, but with the decline of that 
mining town the salt industry was dropped. 
Intrusive Rocks. 
The intrusive rocks of the region are : Dikes, Laccoliths, 
and Volcanic Xecks. The last named group will be considered 
when the subject of volcanoes is taken up. 
Dikes. 
The Plateau-Canyon Creek Dike. — This dike extends 
from the headwaters of Oak creek south to the summit of the 
Apache mountains just west of the Plateau escarpment. It 
is from one hundred feet to a quarter of a mile in width. Its 
north end is composed of hornblende-biotite granite and dior- 
vte. its middle and southern portion of dioryte and rhyolyte. 
At least two eruptive periods, remote from each other, are re- 
presented here: the first occurred in the Algonkian age, the 
second after the close of the Carboniferous, possibly as late as 
the Pliocene period. The lavas of the two periods stand side 
By side in most places, though there are localities in which the 
rhyolyte has removed the dioryte completely from the fissure. 
This dike, therefore, gives evidence that the trachyte ( rhyol- 
yte ) was erupted last, that each eruption was a fissure erup- 
tion, and that the volcanic disturbances were separated by long 
periods of quietude. 
The Salt River dike. — This dike crosses from the north 
side of Salt river to the south side at the salt springs,, from 
which place it trends west parallel with the river. It is min- 
eral bearing, the mineral being a low grade zinc ore. For 
• many miles the dike dips south-southeast, but after passing the 
horse-shoe bend of Salt river it becomes vertical in position 
and farther on takes a northwest dip. 
Along the exposures of this dike at many points west of the 
bend, occur many large volcanic necks or plugs. How far 
these extend beyond the territory examined was not determ- 
ined, but enough is known concerning them to warrant the be- 
lief that they form a prominent feature in the topography of 
the country to the west. 
