LACCOLITHS L\ SOUTJ/EASIERX COLORADO 819 



and southeastern slopes. In neighboring regions the Creta- 

 ceous t\)rmations are characterized by flexures and dislocations 

 of minor importance, and it is probable that a deformation of 

 that general character is here combined with the arching due to 

 igneous intrusion. A computation based on the contours of 

 deformation indicates the total volume of the uplifted or protu- 

 berant mass as a little less than one cubic mile. 



The igneous rocks associated with this local uplift include 

 laccolites and dikes. There are at least two laccolites, and the 

 number may be much larger. The highest is known only by a 

 remnant exposed at various points about the southeastern base 

 of Twin Butte, wdiere it rests on the white limestone (formation 2) 

 and is covered by red shale (3). It does not appear on other 

 sides of the butte, and it is probably limited on the west by a 

 fault. The steep westward dip of the overlying strata suggests 

 that the mass may originally have been large. Beneath it, and 

 separated onlv by the white limestone, is a broad mass whose 

 upper parts onlv are seen. Its outcrop is nearly continuous for 

 three-fourths of a mile from north to south and more than half 

 a mile from east to west. Wherever its relations to the sed- 

 imentaries are seen it passes beneath them, the overlying strata 

 being either formation 2 or some member of formation i. South 

 of this area is a smaller tract of igneous rock which may repre- 

 sent a laccolite or a sill ; and beyond it is the irregular exposure 

 of an intrusive mass which ranges in thickness from fifteen or 

 twenty to more than 100 feet and traverses the formation 

 obliquely so as to be walled in places by formations i, 2, and 3. 

 It is possible that the larger of the observed laccolite masses is 

 the y)rincipal intrusion, occupying practically the whole interior 

 of the dome ; but consideration of the irregularities of the 

 deformation leads rather to the view that the arch includes a 

 number of individual masses. 



About fiftv dikes were noted traversing parts of the dome, 

 and it is probable that others intersect the laccolites. One dike 

 was seen at a point two miles bevond the northwest base of the 

 dome, and at another point outside the dome a well sunk through 



