1896. | NEW YORK ACADEMY OF SCIENCES. 109 
rising some hundreds of feet above the mesa. On the road 
from Madrid, and considerably south from the bore-hole, this 
rock is crossed in an arroye at a higher elevation than in the 
hole, so that there it has burst through the stratified rocks to 
form a dike which is continuous with the hills already referred to. 
The lower sheet extends northward further than does the 
upper, but disappears before the northern face of the mesa has 
been reached, for no traces of it were seen along the road or 
along that face, where the exposures are very distinct. The 
thinning out and abrupt termination of this sheet have caused 
the rapid and abnormal northward dip shown in Coal canon and 
in the northerly levels of the White Ash mine at half a mile 
from the crop, where it is very annoying. 
As already stated, the appearance of these sheets at more than 
one locality is precisely that of typical dikes, but there is no 
possible dispute respecting the intrusive character of the upper 
sheet, still more than 150 feet thick in places, for the slopes of 
mines on the White Ash bed have passed underneath it to its 
eastern limit without crossing any eruptive rock whatever; and 
the lower sheet is equally proved, for in the bore-hole the Waldo 
coal bed occurs, with the Miller’s Gulch coal bed, at the proper 
distance below, increased by the thickness of the trachyte. The 
structure in this coal field is extremely like that in the Anthra- 
cite range of Colorado, as described by Dr. Cross; * the Ortiz 
Mountains being the main mass whence extended the sheets, 
which may have been joined by dikes filling fissures formed dur- 
ing the intrusion of lava. 
The region further south, where gold mining has been carried 
on for many years in a very small way, has always been re- 
garded as badly cut up by-dikes. In the Cerrillos coal fields the 
expected amount of coal was small because of the enormous 
dikes, while in the gold field boring for water was thought to be 
useless for the same reason. But the dikes have proved to be 
sheets in the Cerrillos region and there is good reason for sup- 
posing that they are sheets in the gold field, for several holes 
drilled in 1895 have yielded a good supply of water.+ 
The Laramie Rocks.—The Laramie or highest stage of the 
Rocky Mountain Cretaceous is represented by light yellowish 
to gray sandstones and gray to black shales. For the most part 
the sandstones are deficient in cementing matter and weather 
readily, the thicker beds showing a cavernous surface at the out- 
crop and their fragments assuming a spherical or oval shape. 
* Whitman Cross. The laccolitic mountains groups of Colorado, Utah and Arizona. 
In the 14th Ann. Rep. of U.S. G. S., pp. 157-241. Fig. 31, p. 187. : 
+ For description of the rocks, see notes by Prof. J. F. Kemp, appended to this paper. 
