LACCOLITES, SHEETS AND DIKES. 449 



tive sheets and basement shales have the form of a gently folded synclinal 

 basin which erosion has spared. 



Eruptive Rocks of the northern Area. 



Structural Aspects. — The eruptive rocks are of great interest and novelty. 

 They may be classified for purposes of description as dikes, sheets and 

 Laccolites, without any essential genetic difference. The writer has found 

 no evidence of surface Hows; all rocks appear intrusive and younger 

 than the enclosing strata. 



The dikes are innumerable and occur in every part of the range, vary- 

 ing in widthand position. In the canyons cliffs of horizontal strata may 

 he seen a thousand feet and upward in height, which are intersected by 

 mazes of vertical and oblique dikes. Toward the interior of the range 

 these dikes increase in number. As an example, a dike was counted 

 every fifty feet horizontal on a long spur. 



The sheets are closely connected with the dikes, which sometimes 

 spread out between the strata as sheets, or a sheet may cut obliquely 

 across the strata as a dike to another level. The sheets may be a hun- 

 (1 red feet thick and a mile in extent. It is noticeable that sheets i >ccur on 

 the eastern and western edges of the range where dikes are rarer, and it 

 seems to have been easier for the intrusion to force its way laterally. 

 The facts indicating that many of the sheets have; been folded with the 

 strata after intrusion have been alluded to. 



The laccolites differ from the sheets only in their greater thickness and 

 bulging character. The greatest observed thickness of any one laccolite, 

 free from shale hands, was about -'150 feet, which would be increased to 

 500 if a thin shale parting were omitted. They have a well developed 

 pi-isniatic structure at right angles to the cooling surfaces, and hence the 

 upright columns lean to correspond with the amount of dip. The tilted 

 laccolites are. of course, best exposed, presenting cliffs on one side. The 

 intrusion generally follows the bedding for some distance, hut is liable 

 to cut obliquely across, and without reference to joint planes. In one 

 natural section a long splinter of shale 200 or 300 feet Long ami •"><> feet 

 thick is seen to have keen bent oil' by the splitting of the eruptive mass, 

 hut is still continuous with the shales at one end. 



It is rare to see feeding dikes below the laccolites. They are some- 

 times cut. in common with the shales, by later vertical dikes of the same 

 or different rock which follow joints in the shales. A Limited contact 

 rrietamorphism is produced by the laccolites and thicker sheets at both 

 con i ad-, by which the shales are indurated and often changed to a green 

 color by caustic action. The changes in texture and even mineral com- 

 position produced by dillereiit conditions of cooling in the center and at 



