102 The American Geologist. February, 1902. 
and gravels are loose or very feebly cemented. [iut here and 
there they are consolidated into resistant masses. This in- 
equality of induration aids erosion in producing a great variety 
of erratic topographic forms which give name to Monument 
creek from which the formation derives its name. Agatized 
wood is found in great abundance throughout the formation, 
but no specific determinations have been made. 
14,—Rhyolyte. Above the Monument creek beds lie mass- 
es of rhyolitic tuff. This tuff has been noted by Fiayden and 
others. It forms more or less of a sheet,—or sheets, in some 
places,—while in others, it occurs in somewhat irregular masses 
more or less intermingled with sand, gravel and clay. In 
places the tuff abuts abruptly against gravel beds in a manner 
which is strongly suggestive of an old stream bed filled with 
the tuff. Above the tuff, occurs a sheet of glassy rhyolyte 
about twenty-five feet thick which forms the protecting cap of 
several of the buttes near Castle rock. This rhyolyte is used 
extensively as a building stone in Denver and other cities of 
Colorado. The examination of thin sections shows, according 
to Prof. J. P. Iddings to whom they were submitted that the 
rhyolyte is a flow, but its place of ejection is unknown. 
15,—Castle Conglomerate. The youngest formation in the 
Castle Rock region is a part at least of what has been called 
by Hayden and others the Upper Monument Creek formation. 
The term seems never to have been very definitely applied. As 
used in this paper, the formation consists of the massive con- 
glomerate above the lava, having a maximum thickness. of 
ninety feet, and containing fragments of the rhyolyte obtained 
from the underlying flow. The conglomerate is massive and 
compact, with the constituent parts firmly packed and cement- 
ed. It is composed of coarse, angular, subangular and rounded 
masses firmly set in finer material. The constituents are de- 
rived from the older formations in the mountain regions to the 
west. There are pebbles and boulders of quartz, quartzyte, 
granite, etc., from the crystalline regions, and fragments of 
sandstone, limestone and conglomerate from the older sedi- 
mentary beds. But the distinctive character of this formation 
is the presence of fragments of rhyolyte. These fragments are 
sometimes five feet in diameter though the larger ones are not 
numerous. They usually have sharp or slightly worn edges 
