Stevenson.] 1K)'± [Dec. 7, 



rocks. The length of the section as given is not far from 1100 feet. The 

 extent of change increases with the distance from the median line of the 

 fold. 



A fact observed here is very worthy of note. One of the conglomerate 

 sandstones of the section contains fragments of Quartzite sometimes re- 

 sembling the Silurian quartzites seen elsewhere, but oftener resembling 

 the Carboniferous quartzites of the vicinity. The age of the fragments 

 is unimportant in this connection ; it suffices to know that the metamor- 

 phosis had taken place before the conglomerate in which they occur was 

 formed. On Eagle river, not many miles eastward from Rock creek, the 

 Carboniferous conglomerate contains fragments of Silurian quartzites and 

 the unchanged Carboniferous rocks rest on the wholly changed Silu- 

 rian beds. It appears hardly probable that anything connected with dis- 

 turbance or folding of the rocks caused the metamorphosis of the earlier 

 beds, for there certainly was no upheaval or serious disturbance between 

 the close of the Silurian and the Carboniferous, as the two series are con- 

 formable, though the succession is far from being complete. 



The Dakota sandstones are almost wholly metamorphosed into quarta- 

 ites along the Conejos trail, which crosses the San Juan mountains from 

 Tierra Amarilla in New Mexico to Conejos in Colorado. The most of it 

 is a structureless quartzite. Along the same trail, the Triassic beds are 

 metamorphosed. 



The Dakota sandstone is quartzite on the easterly slope of the Sangre de 

 Cristo mountains in Southern Colorado. 



But in many instances the severest twisting and plication appear to have 

 been without any influence whatever. The Dakota is turned on edge and 

 often fiiulted for a long distance along the easterly foot of the Culebra 

 range of mountains in Southern Colorado and Northern New Mexico, but 

 it shows no change anywhere except where, for a little distance, it has been 

 affected by the proximity of a dike. 



Tlie Middle Appalachian region affords many instances in which shales 

 and sandstones have been subjected to enormous pressure and distortion 

 without any apparent effect. 



Along Clinch river in Russell county of Virginia, in the vicinity of the 

 Clinch fault, the shales of the Knox group are twisted as badly as mica 

 schists are in many localities, yet they show no signs of metamorphism 

 and have not even new planes of cleavage. The Lower or Red Medina on 

 Clinch mountain in the same county is thrown into close wrinkles which 

 aflFect even the harder beds, yet no approach to change is manifest to 

 the eye. Within the same region in Pennsylvania, the conditions are 

 similar. In Bedford county, of Pennsylvania, the Red Medina is thrown 

 into extremely close folds for a distance of more than 1000 feet along the 

 easterly side of Evitts mountain, but no change appears in the rock. At 

 a little way further east are the shales of the Hudson and Utica standing 

 on edge but not showing slaty cleavage. There is, however, a change in 

 the Utica, the black shales, which is noticeable here as well as further east 



