GHENGWATANA SEtltES 



829 



There are color contrasts. All the rocks are dark ;■ some are quite 

 black, fresh, and ringing, while others are brown and dull, and when 

 struck with the hammer develop a reddish powder which is perhaps the 

 normal color of the decomposed rock. 



The conglomerate beds. — Within this Chengwatana series of lava flows 

 are no less than 5 conglomerate beds. The first one, descending the 

 river from the bridge and dam along the south side, is reached in little 

 more than half a mile. This bed is onl}^ 6 feet in thickness and made 

 up of pebbles an inch and less in diameter. Quartz veins are conspic- 

 uous, running in many directions. The pebbles are much decomposed, 



Figure 2.— Concentric Weathering of Chengwatana Diabase. 



yet distinct enough to be recognized as of the volcanic material of this 

 locality. Passing over 10 or 11 flows attaining a total thickness of at 

 least 475 feet, another bed of conglomerate is reached, best seen on the 

 north side of the river. Crossing another lava flow of 30 feet (see figure 

 1), of which 5 feet is a beautiful amygdaloid, a third conglomerate bed 102 

 feet thick is reached. This is the thickest conglomerate in the series. It 

 resists weathering more strongly than the lava flows, as is shown by the 

 prominent ridge which stretches northward from its outcrop until con- 

 cealed beneath the glacial drift. Crossing 6 more lava flows, another 

 conglomerate nearly 50 feet in thickness is found, and soon a fifth, but 



XLVII— BuLT,. Geoi,. Soc. Am., Vol. 12, 1900 



