,^,^n GEOLOGY METAMORPHIC ROCKS OF THE MOUNTAINS. 137 



becomes quite probable wlien one recollects the sbelving- or overlapping 

 which these strata present, or the similar meaning phenomenon of the 

 appearance of the Carboniferous beds from below the Triassic at the 

 north, as well as the great variations of thickness which occur just to 

 the south. At the same time, some of the facts at hand indicate that a 

 peculiar fault, depending on the nature of the sharp fold, and possibly 

 connected with the lava near by, may have caused the present appear- 

 ance. Enough is not yet known to decide with certainty which of these 

 explanations is the true one, but it is hoped that a few^ hours' work dur- 

 ing the coming season may clear up this interesting dynamical problem. 



A few miles farther south of Golden the formation again widens, 

 though not so rapidly as north of the town, and near Mount Yernon it 

 again presents nearly the same section, (No. 14.) The lower dip of the 

 lower beds growing steeper in passing over them eastward till the coal 

 is reached standing nearly vertical, with the flat beds just bej^ond, in 

 Green Mountain, gives the usual type of section hereabouts. Nearly 

 the same order is preserved at Bear Creek, section 15. East of Creta- 

 ceous No. 1 nearl}^ all is here covered, but a few exposures show the 

 upper Cretaceous very nearly level. This space is followed by an out- 

 crop of the coal horizon, standing nearly vertical, and surrounded by a 

 gravel terrace. Beyond, the Lignitic beds again flatten. South, still 

 the same general features appear. The lower sandstones at first occupy 

 a widening zone, owing not only to a lessening dip, but also to an actual 

 thickening of the beds. Near Bradford Hill, section 16, they extend 

 quite a distance upon the schists, having a low dip at the west, (15°,) 

 increasing eastward, (to 40°,) and flattening a^ain in No. 1 Cretaceous, 

 (30°.) As the coal-measures farther out probably steepen, we have 

 again the common type of fold, a larger fold which seems to carry 

 upon it two minor ones, the upper and western one with a rather large 

 radius, the lower and eastern one more abrupt, often setting the beds 

 beyond the vertical ; the larger and main fold, as a whole, being con- 

 cave upward ; the minor folds upon it being convex upward. A large 

 area of apparently subaqueous erosion is an interesting feature here. 

 At the South Platte, section 17, the fold seems to be in a^ simple curve, 

 the dip of the lower red-beds being nearly 70° ; of No. 1, about 50°. 



The abrupt fold which is so constantly present along the outer part 

 of the main upthrow in all the southern half of the district, is probably 

 due to the crumpling which would naturally take place in the upper 

 members of a thick series of rock when folded in a rather sharp curve 

 whose concavity is upward. South from the Platte the mountain fold 

 passes on into the next district. While plication may occur in the 

 plains, it must be to an exceedingly small extent, and wholly subordi- 

 nate to the more profound fold found along the mountain border, the main 

 features of which have just been given. The detail and careful exami- 

 nation of this fold would be rendered quite easy on account of the many 

 cross-cutting streams, and thelessons in dynamical geology to be gathered 

 from it can be of scarcely less interest than the results from the careful 

 study of the historical geology of the region. 



Leaving the plains, the subject next in order is the geology of the 

 mountains. 



CHAPTEE III. 



THE METAMORPHIC CRYSTALLINE ROCKS OF THE MOUNTAINS. 



As was dwelt upon at length in tlie last chapter, the nearly flat sedi- 

 mentary rocks which underlie the plains have an exceedingly well de- 



