March 24, 1899.] 



SCIENCE. 



431 



crest of the main ridge along the southern 

 portion of the range. Probably they are all 

 remnants of one continuous flow. They 

 are best observed when seen eastward from 

 Mountain Basin, when they present a cas- 

 tellated appearance, capping the coarser 

 and lighter-colored rocks. In general habit 

 they resemble the earlier basalt sheets, and, 

 except for their position, have little to dis- 

 tinguish them from other similar flows. 

 The part they plaj^ in the present configura- 

 tion of the plateau is insignificant. The 

 interest lies in the fact that these basalts 

 complete a second cycle of eruption, which 

 built up the Absarokas by the accumula- 

 tion of successive flows of extrusive lavas, 

 and that with them, so far as we have any 

 positive record, the last phase of a long- 

 continued series of eruptions came to a 

 close. 



That the piling-up of this eruptive ma- 

 terial lasted through a long period of time 

 is clearly established. In the first place 

 the earljf acid breccias show evidences of 

 considerable denudation before the pouring- 

 out of later lavas which now occupy the 

 eroded areas. Not infrequently depressions 

 may be seen filled with water-laid silts and 

 fine gravels, which were afterwards covered 

 by fresh outflows of breccia. Similar water- 

 laid deposits may be observed in all the 

 breccias, but they especially characterize 

 the eai-ly basic series along the east side of 

 the range, where the former existence of 

 large lakes and ponds is manifest, with 

 sediments of volcanic material over 200 

 feet in thickness deposited in comparatively 

 quiet waters. In certain localities the 

 basalts appear to be the result of fitful dis- 

 charge and slow building-up from numer- 

 ous vents. The thinning and thickening of 

 beds in various directions, the overlapping 

 of thin beds from different centers, and 

 the frequently chilled surfaces of vesicular 

 basalt, all point to a slow accumulation of 

 the ejected lava. Occasionally in basaltic 



cliffs between lava sheets may be seen thin 

 beds of volcanic sands and gravels, wind- 

 strewn over an exposed surface before be- 

 ing buried beneath fresh flows. Nowhere 

 were interbedded layers of clay or earthy 

 beds of decomposed rock observed, but such 

 deposits are, I think, exceptional in most 

 basalt areas. 



While the gradual building-up of the 

 plateaus from fresh accumulation was 

 steadily in progress, erosion was constantly 

 at work upon the surface ; and, although 

 volcanic fires ceased long ago, erosion has 

 been going on steadily ever since. One of 

 the most remarkable and puzzling features 

 of the country are the areas of undoubted 

 water-worn volcanic material, with its 

 smooth and polished bowlders. Accumu- 

 lations through floods and freshets abound ; 

 and, besides the evidence of ancient lakes 

 and ponds found dotted over the surface, 

 there are strong grounds for the belief that 

 upon the upland existed broad rivers which 

 carried the water-laid material across the 

 plateau to the plain below. All this re- 

 quired a long time for its accomplishment. 



Turning now to the land vegetation, con- 

 vincing arguments are found not only for 

 determining the age of the rocks, but for 

 demonstrating that the eruptions lasted 

 throughout a long continued period of ac- 

 tivity. It is doubtful if any other known 

 region in the world offers such a promising 

 field of research, showing the relationship 

 between plant life and volcanic eruptions, 

 as is to be found in the Absarokas. In 

 solving these problems the geologist is 

 greatly indebted to the paleobotanist. From 

 time to time extensive collections of fossil 

 plants have been made, indicating a rich 

 and varied flora. Portions of the region 

 have been visited by our distinguished fel- 

 low rnembers, Professor L. F. Ward and 

 Professor F. H. Knowlton. All of the col- 

 lections have been referred to Professor 

 Knowlton, who has made an exhaustive 



