OCCURRENCES AND CHARACTERISTICS. 227 



both banks of this branch of Fountain creek for two miles or more 

 above the forks. On the northeast side is one of the largest dikes, form- 

 ing a sharp and very jagged ridge more than half a mile long. On the 

 opposite hillside numerous small dikes are exposed, but the most excel- 

 lent outcrops are about one and one-half miles above the forks of the 

 gulch, where its course is east and west, on the northern side. Here 

 several bands of small dikes are found, some forming wall-like projec- 

 tions, while the ramification is very plain. 



' Another excellent exj^osure of dikes has been made by a railroad cut 

 one and one-half miles southwest of Woodland Park, on the west side of 

 a little divide. Here the road turns abruptly to the south around a hill 

 of red quartzite caused by a large dike, whose contacts are not well 

 shown, except in the cutting on the west side, at which place the main 

 contact is exposed, and also a dozen or more small dikes, within fifty 

 feet of the large one. The relationships are ver}'' clear at this point. 



The largest dike is one and a half miles due west of Woodland Park. 

 It fornjs a ridge several hundred feet high, of very rugged outline, and 

 its maximum width was estimated at over 300 yards, though the exact 

 contacts were covered by debris at the points visited. This dike extends 

 northward for a mile or more with decreasing thickness and probably 

 divides into several branches. It is not known how far the dikes extend 

 to the northward. 



The Dike-rock. — The rock of the dikes is a fine and even-grained aggre- 

 gate of sand grains, variously indurated. In some cases it has the saccha- 

 roidal texture seen in many massive sedimentary sandstones, but it is 

 more commonly indurated to a dense, hard quartzite. Except for a local 

 and subordinate banding adjacent and parallel to the walls of a few dikes, 

 the rock, even of the largest dikes, is remarkably massive and uniform 

 in character from wall to wall. A dull red color, of various shades in 

 different dikes and due to flakes of limonite, is characteristic of the 

 masses. 



The rock is extremely tough, as a rule, and the breaking down of the 

 dikes is chiefly due to a jointing, which is generally developed parallel 

 or at right angles to the walls, but may be quite irregular. This jointing 

 produces debris of small, shar})ly angular fragments, which, as talus, con- 

 ceals the contacts of the larger dikes in most places. 



The constitution of the dike-rock is evident to the naked eye in most 

 cases, and a hand-lens shows the character of the densest masses. Micro- 

 scopical examination simply demonstrates the extraordinary purity of 

 the sand, showing that it consists almost wholly of quartz grains, many of 

 which are well rounded, while few are sharply angular. There is a very 



