LAccoi.rri:s ix sor'rf/i'.Asrj-'.Rx Colorado 821 



r^ran'incnts of various rocks are included in the laccolites and 

 dikes, and arc of interest as revealing" the nature of lower-lying 

 terranes through which the ascending liquid passed. Besides 

 sandstones and shales similar to those constituting the wall rocks, 

 the most abundant as well as the most notable rock is a porphy- 

 ritic granite with conspicuous crystals of gray feldspar. 



The age of the laccolitic intrusion is not closely determined. 

 The \oungest formation involved in the deformation is the 

 Dakota. The Neocene sand rests undisturbed on the worn edges 

 of the deformed strata. Manifestly the intrusion was subsequent 

 to the one and antecedent to the other. These limits stand wide 

 apart, but a little consideration will show that the epoch of 

 intrusion was probably not close to either. In discussing the 

 laccolites of the Henry Mountains, the writer reached the tenta- 

 tive conclusion that a heavy load of overlying rocks was a con- 

 dition essential to the formation of a large laccolite, and the 

 body of data which has since been gathered is rather confirma- 

 tory of this conclusion than otherwise. If this be admitted we 

 must assign to the intrusion a date at which the Dakota sand- 

 stone was covered to a great depth by other formations. It is 

 known from the general history of the region that the deposition 

 of the Dakota was followed by that of the Colorado and Mon- 

 tana groups, and it is possible that these were here succeeded by 

 the Laramie also. Subsequently all these beds were eroded, not 

 only from this particular district but from an extensive tract of 

 the Great Plains province, and much time was necessarily con- 

 sumed in this work. It seems therefore probable that the date 

 of the igneous intrusion belongs either to the closing epochs of 

 the Cretaceous period, or to the earlier half of the Eocene 

 [period. 



The porphyrites and cognate rocks which elsewhere consti- 

 tute the greater number of laccolitic masses are notable for 

 their durability. Not only are they so hard as to resist corra- 

 sion stubbornlv and cause the diversion of such small streams 

 as mav chance to flow athwart them when they are discovered 

 bv the general degradation of the countrv, but they yield with 



