INTRUSIVE ROCKS. AQ 
which is entirely altered to epidote, with some associated quartz. As 
there are in the same section feldspars having the more usual alteration 
above described, this must be a different variety, probably a plagioclase 
very rich in lime—anorthite (?). Quartz is common as a decomposition 
product of the phenocrysts, and occasionally becomes so prominent as to 
make up the larger portion of the pseudomorph. In one section, however, 
a quartz crystal, which appeared to be original, was found among the 
pseudomorphs. A constant and striking mineral, apparently original, is 
ilmenite, which in the sections examined is always present in remarkable 
profusion. This has crystal form and shows cleavage. It alters occasion- 
ally in a limited degree to a blood-red, translucent oxide—red hematite (?)— 
but usually to the milky opaque alteration product known as leucoxene. 
Magnetite also occurs in crystals as an original constituent; pyrite occurs 
sporadically, and is probably secondary. The groundmass in which these 
minerals are set is moderately fine grained and granular; its constituent 
minerals are chiefly feldspar, both orthoclase and plagioclase, and quartz. 
Porphyry dikes —In two localities in the district examined small dikes of 
greatly decomposed rock, which, however, appeared to be the same as the 
rock of the more persistent sheet, were found. One of these localities is in 
the Tourtelotte Park special area, where a small dike was noticed in the 
Silurian dolomite, above the main sheet; the other is on Maroon Creek, 
where a dike a few feet wide was seen cutting the Triassic sandstones. 
Source —The marked thickening of the sheet of porphyry toward the 
south and its disappearance toward the north point out the direction from 
which the intruding rock was propelled. From the southern edge of the 
district in which detailed mapping was carried on the sheet was not continu- 
ously followed southward, but at several points toward the south it was 
observed, always thickening, and it undoubtedly runs into the great diorite 
mass of Castle Peak, some 10 miles away. Castle Peak is made up mostly 
_ of complex dikes and intercalated sheets of eruptive rocks in the Maroon 
Carboniferous beds. To the south this complex soon changes to the solid 
diorite, as shown in the adjacent White Rock Mountain, which is a part of 
the great cross-cutting body of diorite found all along the axis of the Elk 
Mountains, and whose advent was one of the chief phenomena connected 
with the formation of that range.’ 
'Whitman Cross, Fourteenth Ann. Rept. U. S. Geol. Survey, Part IT, 1894, p. 179. 
