no Bulletin of Laboratories of Denison University [Voi. xit. 
This flow proves to be a characteristic lencite phonolite. 
From this point to the western foot of the Cerrillos mountains 
the same fossiliferous shales and flags occur in low hills. Essen- 
tially the same sequence occurs south of the creek. The region 
to the west and south of the mountains are cut by radial dykes 
which beautifully illustrate the way in which these dykes habit- 
ually occur. The intersected rock being in this case for the 
most part soft and easily eroded, the dykes are left as conspicu- 
ous walls rising several feet above the general level, and may be 
traced for six or eight miles continuously. They vary from two 
or three feet to twenty feet in width and the same dyke may 
vary greatly in this respect. The material, so far as can be de- 
termined without a section, may be an andesite but in the 
thicker portions it tends to be replaced by a felsitic material. 
This may be explained upon the supposition that where the 
local metamorphism was great the fused acid material expanded 
the original fissure by disintegrating the walls and, in some in- 
stances nearly or quite suppressed or obscured the eruptive. 
The alteration phenomena in proximity to the dyke are 
quite interesting. In particular, the formation of chalcedony 
and jasper from the siliceous wall rock is plainly seen. The in- 
fluence of the sheets of the same material, while less pro- 
nounced, is no less apparent, for the shales and limestone so 
covered have become flectuously lammelate and crystallized. 
The source of the flows as well as of the dykes is no doubt the 
south end of the Cerrillos mountains which are composed of a 
similar material. The northern part of the range is of granitic 
rock which may be later than the old phonolite nucleus. This 
region gives us the suggestion that the phonolites are probably 
post-cretaceous. To the northwest of the mountains, about 
three miles from their base, is a recent basaltic crater from 
which extensive flows of lava have spread to the west and south- 
west. Along the southern margin of this sheet there has been 
very extensive base-leveling erosion exposing from 50 to 75 
feet of the soft strata beneath. In the vicinity of the mountain, 
along its western base such exposures are very instructive and 
give the surest clue we have so far seen to the age of the several 
