226 W. CROSS — INTRUSIVE SANDSTONE DIKES IN GRANITE. 



Characteristics and Mode of Occurrence. — The dikes have a general trend 

 parallel to the belt in which they occur. They stand vertical or have a 

 steep dip to the northeast, and often appear as a complex of nearly par- 

 allel fissures with many branches and connecting arms. In width they 

 vary greatly. The larger number are a few inches or a few feet thick, 

 but many of the smaller branches thin out to a mere film. On the other 

 hand several dikes are many yards wide, and two form prominent ridges 

 with a width of from two to three hundred yards each. 



The length of the dikes cannot be accurately determined, for the sur- 

 rounding granite is so much disintegrated by weathering that its gravel 

 covers and conceals the dikes wherever they are broken down. The 

 largest ones, however, may be followed for nearly a mile, and they are 

 doubtless directly connected with some of the smaller ones whose out- 

 crops are interrupted by the granite gravel. All evidence tends to show 

 that the dike fissures belong to a single system and that they are con- 

 nected at jDoints not now exposed. Certain bands of small dikes are 

 visibly connected by so many cross-fissures that the granite is locally 

 divided into wedges or angular blocks. The figures of the accompanying 

 plate, representing two faces of the same specimen, illustrate the char- 

 acter of the fissures and their complicated branching, such as may fre- 

 quently be observed between the larger dikes of a complex. 



The larger dikes form ridges with narrow crests rising abruptly three 

 or four hundred feet above the parallel gulches. The hard and much 

 jointed dike-rock causes very rugged forms contrasting markedly with 

 the gently sloping hills of granite about them. Dikes a few^ feet wide 

 often stand out as walls above the gravel surface. 



The smaller dikes have walls which are apparently parallel planes and 

 the contacts are very sharply defined. The larger ones have a fiat len- 

 ticular shape in horizontal section. In all formal relationships to the 

 enclosing rock these bodies are as typical dikes as any of igneous origin. 



As the best exposures of these bodies are easily accessible from either 

 the Woodland Park or the Green Mountain Falls station on the Colo- 

 rado Midland railroad, some further details of their occurrence will be 

 given, for the benefit of those who may have opportunity to visit this 

 district. At a point one and one-half miles above Green Mountain Falls 

 the main western branch of Fountain creek issues through a narrow 

 gulch in granite hills into the southern end of the long depression occu- 

 pied by the sedimentary rocks of Silurian and of Carboniferous (?) age, 

 which extend from this point northward for eighteen miles through 

 Manitou park.* The sandstone dikes are most beautifully shown on 



_ _ __ ___ — _ , . ^ 



*See Hayden's Atlas of Colorado, sheet xiii. 



