438 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
stoped-out Portland vein, but as these old upper levels are for the most part inac¬ 
cessible, it is not at present practicable to ascertain the form and extent of this 
eruptive mass. It has not been recognized on the 350-foot level, but on the 500- 
foot level a long crosscut northeast of No. 2 shaft passes for about 400 feet through 
a body of altered massive rock which is probably part of the same mass (PI. 
XXVIII). Two other levels, the 500-foot and the 600-foot, reach this latite- 
phonolite. On the 500-foot level a massive rock is exposed on the south side of the 
station of the No. 2 shaft. It is too much altered for satisfactory determination, but 
is apparently latite-phonolite. A similar altered rock occurs along the east side of a 
little drift 450 feet northeast of the Burns shaft (PI. XXVIII) and is probably part 
of the same mass. On the 600-foot level what seems to be the southern continuation 
of the same body of rock is encountered in the north face of a short drift about 500 
feet east of the Burns shaft, and apparently also in a crosscut about 200 feet east 
of the No. 2 vein. The rock on this level is in part unaltered and is clearly a latite 
phonolite. (See p. —.) It might have been expected that the same rock would 
have been cut in the long northeast (Last Dollar) crosscut on the 800-foot level. 
But this shows nothing but breccia, and it is probable that the latite-phonolite of 
this part of the mine occurs in masses of very irregular shape. 
All the large rock masses of the Portland mine are cut by numerous dikes. 
These exhibit considerable diversity of trend, but the greater number have courses 
ranging from north to N. 35° W. In other words, dikes and lodes are generally par¬ 
allel or coincide. The general arrangement of the dikes is illustrated for one of the 
most important and representative levels in PI. XXVIII. 
The most abundant dikes are those of phonolite. They range in width from 
a few inches to 30 or 40 feet, the greater number, however, being from 2 to 5 feet. 
The phonolite is invariably altered, and presents the usual appearance character¬ 
istic of this rock when occurring as dikes within the mineralized portion of the dis¬ 
trict. It is usually a pale-green aphanitic rock with a well-marked platy parting 
parallel to the dike walls. While many of the phonolite dikes are very irregular, 
changing in width, dip, and strike from point to point, some maintain a fairly 
uniform width and trend for over 1,500 feet. One of the most persistent is a nearly 
north-south dike which coincides with the No. 9 Captain vein on the north, passes 
just west of No. 2 shaft, and continues into Stratton’s Independence mine about 
600 feet southeast of the Burns shaft. It is not certain that this is really a single 
continuous dike, but it has been cut and drifted on at so many points (PI. V, p. 26) 
as to render such continuity probable. As examples of shorter and more irregular 
dikes may be cited some occurring near the No. 2 shaft on the 500-foot level and 
the dike accompanying the Portland vein. 
Besides the phonolite dikes, there are a number of “basalt” dikes. These are 
usuallv nearly vertical and have the same general strike as the phonolite dikes. 
Tliev are rarely over 6 feet in width, and fairly regular. When followed by drifts, 
they are frequently found to pinch and disappear, but a short crosscut to one side 
or the other w 11 usually reveal a second dike, which when followed soon attains 
the average width of the first and continues on the same general course. These 
dikes are invariably decomposed, so that their original petrographic character can 
