280 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
Minnelusa (upper Carboniferous) formation, to the north and east of 
Whitewood Peak (Plate 2, fig. 1). This porphyry differed from that 
of the main laccolith in that here the phenocrysts were quartz-crystals 
in perfect di-hexagonal pyramids, instead of slightly rounded feldspars, 
as in the main mass of the porphyry. On the west side of Whitewood 
Creek, opposite the mouth of Sandy Creek and in the bed of the stream 
as well, is a large mass or stock of porphyry breaking through the 
Minnelusa. (Plate 3.) 
The sills found in the Minnelusa to the north of Whitewood Creek, 
on the crest of the dome of the laccolith, were all very thin, never more 
than a few inches thick, and of no great extent. They were best seen 
in the small railroad cut near the entrance to the tunnel, north of the 
sharp elbow made by the Creek. The sills of the region around Sandy 
Creek were, as can be seen from the map, of great extent, and one at 
least was about a hundred feet in thickness. 
The Minnelusa formation is composed of sandy limestone and sand- 
stone in rather thin beds and overlies several hundred feet of massive 
Mississippian and Ordovician limestones. These latter are resistant, 
and, while bending to accommodate the new conditions of a laccolith 
beneath the Cambrian, are too compact to allow any porphyry sills to 
spread out in them. The Minnelusa, on the other hand, being thin 
bedded and composed of beds of differing texture, is particularly well 
adapted to allow the formation of sills. 
The size of these sills in different parts of the area is evidently 
dependent upon the laccolithie dome. In the syncline between the 
two laccoliths of Whitewood Canyon and Crook Mountain to the 
northeast the bending of beds on different radii gives large open 
spaces which offer a locus for the intruding porphyry. Moreover, 
the cracks forming as the result of the synclinal bending would gape 
downwards, thus allowing more porphyry to enter and enlarge the 
open spaces already present in the syncline. This condition is some- 
what analogous to ore deposits of the type of Broken Hills (N. S. W.) 
where the open spaces in a syncline have been filled with mineralizing 
agents instead of porphyry. On the crest of the anticlinal dome, how- 
ever, a different set of conditions would be present. The beds, being 
of different textures and thicknesses, would bend to curves of different 
radii, as in the syncline, but the weight of the strata above the arch 
would tend to flatten out the open spaces, leaving less room for the 
formation of sills. Also the cracks formed, although they might be 
more numerous than in the syncline, would gape upwards and thus 
not allow porphyry to enter. 
