Stevenson.] lOb [Dee. 7, 



the purer sandstones into quartzite and the micaceous sandstones, thus 

 altered, so resemble granite that one examining only a hand specimen 

 might well hesitate before deciding the name of the rock. The miners in 

 the region stoutly maintain that this is granite. 



Intruded sheets of basalt have converted coal into coke in Purgatory, 

 Dillon and Upper Canadian canons in the Trinidad coal field of Colorado 

 and New Mexico. On the northerly slope of the Placer mountains in 

 New Mexico, an enormous dike of trachyte has converted a bed of coal 

 into anthracite for a distance of certainly one-fourth of a mile. 



But in many cases the effect is imperceptible. Along the Upper Ar- 

 kansas in central Colorado, especially in its great caiion passing through 

 the east side of the Sangre de Cristo mountains, known further north as 

 the Park range, numerous instances of contact between eruptive and sedi- 

 mentary rocks were observed. The Carboniferous rocks dip very sharply 

 near Pleasant valley and an overflow of basalt rests on their upturned edges. 

 There seems to be no alteration at the line of contact. At a little way 

 further up the river, an enormous dike breaks through the same sandstones 

 and overflows broadly ; the line of contact is well-shown on each side. 

 Careful examination disclosed no perceptible alteration in the sandstones. 

 At several localities within this canon, the great sheet of lava, which ex- 

 tends from the eruptive area at the south-west almost continuously into 

 South Park, is frequently seen following the eroded surfaces of the sand- 

 stones and coming down almost to the level of the road. Many oppor- 

 tunities occur for examination of the contact, but one rarely finds the 

 alteration extending to more than a very few inches. These rocks are mica- 

 ceous and contain a large proportion of argillaceous matter. 



On Mount Lincoln, near South Park in Central Colorado, a great dike 

 can be seen for nearly 3500 feet above the timber line, passing dii'ectly 

 through the Silurian and Carboniferous beds. The effect on the Silurian 

 cannot be determined as some other agent has metamorphosed those beds 

 throughout. The change in the Carboniferous is insignificant. The lime- 

 stone is altered slightly in color and is somewhat brittle, while the sand- 

 stone has become an imperfect quartzite. In each the change is percepti- 

 ble for a very few feet. 



In Dillons canon. New Mexico, a sheet of basalt is shown between 

 sandstones at a mile from the canon's mouth. No eflect whatever has 

 been produced on the character of the rock, though vapor holes are pres- 

 ent, showing the intense heat ; yet at a short distance further up the canon 

 the same sheet has converted into fine quartzite the sandstone on which it 

 rests. 



In the Upper Canadian canon a sheet of basalt has converted a coal bed 

 into coke, but another at a few feet higher up the hill has had no influ- 

 ence whatever on the rock in contact with it. 



On the northerly slope of the Placer mountains a narrow dike of basalt 

 cuts through the Laramie beds, its course being fully exposed for 38 feet, 

 including two thin beds of coal. No eflfect whatever has been produced 

 on the coal even at the line of contact. 



