108 TRANSACTIONS OF THE [saNn. 20, 
the mesa, but increasing rapidly in the side canons, where it has 
suffered less from erosion. Further down, especially near Mad- 
rid, irregular erosion of the wall shows that this sheet dips with 
the stratified rocks. Its northern limit is at the road leading 
from Madrid to Ortiz caion, whence northward the mesa is 
lower. Following that road beyond its summit, which is at the 
westerly edge of the mesa, one finds an unbroken mass of the 
trachyte south of the road, extending almost to Ortiz canon and 
apparently continuous southward to the Ortiz Mountains, all 
overlying beds having been removed by erosion. As far as could 
be ascertained, only a narrow dyke-like strip, not more than 70 
feet wide, extends northward beyond the road and terminates 
near the bluff overlooking the White Ash pit. The mass south 
from the road is interrupted in a petty cafion and in like man- 
ner by Ortiz canon itself, Laramie rocks being exposed in both. 
It is certainly thinner in these canons, for coal beds are shown 
in the former at several yards higher than an exposure of the 
trachyte little more than 100 yards further west. There seems 
to be good reason for believing that the sheet broke across the 
strata near Ortiz cafion so as to rest on higher beds there than 
in Coal cafion. 
Erosion has cut this sheet into long strips between Coal and 
Ortiz canons so as to make it resemble a series of dikes—a re- 
semblance increased by the abrupt change in course at a little 
way west from Ortiz canon. But the slopes of the White Ash 
and Lucas mines have passed underneath this mass, the former, 
now 3100 feet long, extending to within a few feet of Ortiz 
canon, and, therefore, practically beyond the limit of the sheet. 
A very distinct dike, 4 feet wide, was seen in a little side 
canon just below the old Boyle mine on Coal canon. It passes 
directly into the overlying mass, but its northward extent could 
not be ascertained. Another, fully 12 feet wide, is shown at 
Madrid, near the Company’s store; it passes through the village 
and soon becomes a conspicuous feature northward, along the 
westerly side of Coal canon. These are distinctly dikes, with 
the stratified rocks in direct contact on both sides; but they are 
evidently branches from a deeper sheet, which does not come to 
the surface along Coal cafion. The broader dike does not reach 
to the upper sheet near Madrid, for it has not been crossed in 
the Lucas mine. 
Two years ago the coal company drilled a hole, beginning on 
the mesa west from Coal cafion. The lower sheet of trachyte 
was reached at about 200 feet from the surface and proved to 
be almost 200 feet thick. It outcrops in a shallow cafion at the 
west, along which it can be traced southward into rugged hills 
