IGNEOUS FORMATIONS. 281 



from the wiills inward, as is often found to have been the case in dikes 

 where a pronounced transverse columnar structure is visible. The more 

 uniform cooling may be considered natural it' we assume the dike to have 

 been the channel through which large volumes of lava passed to the 

 surface, thus heating up the adjacent strata before the final cooling began. 



The strata traversed by the dike do tiol seem to have been much 

 altered l>v the basalt, but the contact is so generally covered l>v debris 

 that little direct evidence on this point is obtainable. However, it would 

 be contrary to experience elsewhere in this district if any pronounced meta- 

 morphism should be found <>n the contact of the basall and sedimentaries. 



The walls of many fissures are coated by calcareous deposits, indicat- 

 ing former spring action. At the mound forming the eastern extremity 

 of the dike the edges of the rocks are rounded and worn, apparently by 

 the spring waters which deposited the white tufaceous matter now filling 

 the wide cracks. 



llavden and Marvine refer to this dike in the Annual Report of the 

 Hayden Survey for 1873 1 and a sketch showing the west end is given, 

 but the statements are somewhat inaccurate. 



THE KA1.STON DIKE. 



The second dike of importance is situated about 2 miles northwest of 



the northern edge of 'Faille .Mountain, and 1 I miles a little west of south 

 from Valmont. The dike rises abruptly on the south hank of Ralston 

 ('reek, about halt' a mile west of Murphy's coal mine, and extends for 

 more than 1 mile nearly due south. Facing the creek the dike exhibits 

 a broad (dill' more than L,000 feet wide and :;:><) t<» 500 feet high, with 

 rude vertical columnar structure in places. Basalt is seen in place nearly 

 down to the creek bed, while on the north bank are the Cretaceous shales 

 and cla\s, and two or three narrow dikes of basalt, none exceeding n' feet 

 in width, which can lie traced to the northwest for nearly 1 mile, where 



they disappear under the drift terrace. Excepting these comparatively 

 insignificant offshoots, no basalt is known on the north side of the creek 



To the south the dike seems to ) >e doidile, for there are two nearly 



Ann. Rept. U. S.6eol.and Geog. Snrv.Terr. for 1*7:',, pp.29, 130. 



