434 



SCIENCE. 



[N. S. Vol. IX. No 221. 



to the great value of tlie fossil flora as an 

 aid in deciphering the geological history of 

 the Absarokas. Its interest and impor- 

 tance cannot be overestimated. 



Only brief allusions have been made as 

 yet to the intrusive bodies, although they 

 play a most important part in the build- 

 ing-up of the Absarokas. Although such 

 bodies in the form of dikes probably cut the 

 breccias from time to time, it is clearly 

 evident that all the large intrusions, to- 

 gether vfith the greater part of the dikes, 

 were forced upward and into the breccias at 

 two well-defined periods of eruption. The 

 first of these periods was in part contem- 

 poraneous with the early basalt flows, and 

 in part followed them. The second fol- 

 lowed the late basic breccia and basalts, 

 and, so far as can be told, completed the 

 final chapter in the geological history of 

 the immediate region. It is possible that 

 later eruptions took place and that the 

 material ejected was removed by erosion, 

 but of this there is no positive record other 

 than a few isolated patches of rhyolite 

 which do not bear directly upon the prob- 

 lems before us and which may be regarded 

 as outliers of the rhyolite of the Park 

 plateau. It does not follow that the in- 

 trusions of either period were contempora- 

 neous in age, but simply that they belong 

 to a certain phase of the eruptive energy. 

 Dikes may cut an earlier series of intru- 

 sives, and subsequently other dikes may 

 intersect those which preceded them. 



For the purpose of clearly discriminating 

 between these two groups of rocks, the one 

 that followed *the early basic breccia has 

 been named the Sunlight intrusives, from 

 their remarkable exposures along Sunlight 

 Creek and valley, while the later group 

 has been named the Ishawooa intrusives, 

 from the canyon of that name, where the 

 complexity of their occurrences forms one 

 of the most striking features of that impres- 

 sive gorge. In mineral composition the 



Sunlight intrusives range from a quartz- 

 augite-andesite, through transition forms 

 of syenite and diorite, to orthoclase-gabbro. 

 The large body at the head of Sunlight 

 Creek is mainly a syenite with associated 

 monzonites and diorites. On Closed Creek, 

 in Crandall Basin, the intrusive body con- 

 sists for the most part of orthoclase, gabbro 

 and diorite. The series as a whole shows 

 an association of the minerals augite, plagio- 

 clase and orthoclase, with quartz and 

 biotite in its more siliceous members, and 

 olivine and hypersthene in its basic mem- 

 bers. In general, the Sunlight intrusives 

 are more diversified in chemical composi- 

 tion than those of the Ishawooa group. 

 The latter are more siliceous, carrying less 

 of magnesia and alkalies, and the coarsely 

 crystalline masses are much more like 

 normal diorite, diorite-porph3'ry, granite, 

 granite-porphjrry and andesite-porphyry. 



Of the Sunlight intrusive bodies the one 

 situated near the source of Sunlight Creek, 

 in the central portion of the range, is the 

 most impressive, and at the same time the 

 most typical in its occurrence. It measures 

 nearly 3 miles in length and occupies the 

 basins of all the deep glacial amphitheatres 

 on the north side of Stinkingwater River, 

 while all the high intervening ridges sepa- 

 rating the basins consist of indurated 

 breccia. Similar rocks are exposed in the 

 Silvertip Basin, on the south side of the 

 peak, and in all probability they form part 

 of one continuous body. 



The Crandall Basin stock, under Hur- 

 ricane Jlesa, exposed by the erosion of 

 Closed Creek, has far less lateral ex- 

 pansion, but rises for nearly 3,000 feet 

 above its base. Dikes radiate from all 

 the large intrusive bodies, but nowhere else 

 is their number so great and the part they 

 play so strongly marked as in the region of 

 the Sunlight stock. These dikes are by no 

 means all connected with the large stocks 

 seen at the surface, but may be observed in 



