M. R. Thorpe— The Abajo Mountains, Utah. 389 



insinuating itself between and doming the strata to form 

 a laccolithic chamber. From all of the data at hand, it is 

 justifiable to call the Abajo Mountains a group of lacco- 

 liths, coming under the head of injected bodies in Daly's 

 classification of intrusive igneous masses. 



Age of Intrusion. 



There is an unanimous opinion among all students of 

 the laccoliths of the Southwest that they are Tertiary in 

 age, most probably Early Tertiary in the Late Laramie 

 time, and the Laramide Eevolution was responsible for 

 their intrusion. There are certain structures, such as 

 the Comb Monocline, of pre-Tertiary age in this area and 

 these have been magnified by the intrusions. Hence the 

 Abajo laccoliths were formed subsequent to these ante- 

 cedent structures and hence can not be earlier than Ter- 

 tiary. Again the intrusions took place subsequent to the 

 deposition of the Middle Cretaceous and there seems to 

 be no valid reason to doubt that all of the Upper Creta- 

 ceous likewise covered the area at the time of intrusion. 

 Further than this it is impossible to date the intrusion 

 due to the lack of any Tertiary deposits over this area. 

 The age of the individual laccoliths, sheets and dikes, with 

 respect to one another, could not be determined as they 

 were not observed cutting each other. 



New Haven, Conn. 



