Stevenson.] 10^ [Dec. 7, 



minishes to the top of No. 6, that and the underlying stratum being wholly 

 unchanged, so far as the eye could determine ; above these the change is 

 distinct, for Nos. 2, 3 and 4, are perfect quartzites so far as they are ex- 

 posed. The agent which can convert sandstone into structureless quartz- 

 ite does not suffice to effect much change in limestone, for the limestones 

 of the section are rendered merely brittle, there being no trace of crystal- 

 line structure. 



Rocks of the same age at the head of West Fork of Taylor river, also 

 in central Colorado, show a somewhat similar condition : 



1. Quartzite, black. 



2. Quartzite. 



8. Shales, somewhat arenaceous and almost unchanged. 

 4. Quartzite, structureless. 

 5 Sandstone, little changed. 



6. Quartzite, on top completely changed ; middle, the change 



is slight ; but at base the quartzite is structureless. 



7. Granite. 



The total thickness of section is not far from 250 feet. The conditions are 

 more marked here than in the previous section, for in No. 6, which appears 

 to be a continuous mass, the change is least in the middle. No. 5 is a 

 thick stratum lying between thicker ones, which have been wholly 

 changed, yet it seems to be wholly unchanged. 



The unchanged rocks in each section are more or less argillaceous. 



The Cretaceous rocks in South Park, Colorado, show some interesting 

 diflferences, though the conditions appear to be precisely the same. On the 

 west side, where the South fork of the Platte river breaks through a low 

 ridge, the sandstone and conglomerate of the Dakota rest on gneiss and 

 show no evidences whatever of having been subjected to any metamorpho- 

 sing agent ; on the east side near the Sulphur Springs, the same beds are 

 found in the same relative position, but entirely changed, the conglomerate 

 being converted into remarkably beautiful quartzite. The Colorado lime- 

 stone on this side has been changed in color and it breaks along well de- 

 fined planes. The Dakota beds are shown west from the main divide at 

 the junction of Taylor and East rivers, where they are resting on gneiss 

 and are wholly unchanged. 



The Carboniferous rocks rest directly against the Archaean at the head 

 waters of the Purgatory river in Southern Colorado, but no evidence of 

 metamorphism appears there, which is perceptible to the unaided eye. So 

 also on Cebolla creek, a tributary to Mora river in New Mexico, limestone 

 of Carboniferous age is seen resting on granite and wholly unchanged ; but. 

 on Manuelitos creek, only a few miles away, the following section is 

 shown : 



1. Sandstone and shale, wholly unchanged. 



2. Sandstone becoming quartzite below and passing imper- 



ceptibly into 



