SURVEY OF COLORADO AND NEW MEXICO. 29 



deepest interest to the .student of geology in this country, and one 

 around which there is no small degree of obscurity. The effects are 

 universal, however, the evidences of erosion and the worn drift mate- 

 rials being found on the summits of the highest ranges as well as in the 

 lowest valleys, and each district pointiug out the source of these eroding 

 and transporting agencies in the immediate vicinity. 



Since leaving St. Train Creek, the tertiary beds containing the coal 

 have been aproaching nearer the mountains. North of this point the 

 belt of cretaceous rocks has been quite wide, varying from two to five 

 miles, but in the valley of the Boulder the belt becomes quite narrow, 

 and forms a part of the foot-hills themselves, while Nos. 4 and 5 are 

 entirely concealed from view. 



In the Boulder Yalley the tertiary coal beds are enormously developed. 

 The Belmont or Marshall's coal and iron mines, on South Boulder Creek, 

 are the most valuable and interesting, and reveal the largest development 

 of the tertiary coal-bearing strata west of the Mississippi. 



In the autumn of 1867 I had an opportunity of examiuiug these mines, 

 under the intelligent guidance of J. M. Marshall, esq., one of the owners 

 of this tract of land, and I wrote out the results of my examinations at 

 that time in an article in Silliman's Journal, March, 1808. In July, 1869, 

 I made a second examination of this locality under the same auspices. 

 The following vertical section of the beds was taken, which does not 

 differ materially from the one hitherto published : 



48. Drab clay with iron ore along the top of the ridge. 



47. Sandstone. 



46. Drab clay and iron ore. 



45. Coal, (No. 11,) no development. 



44. Drab clay. 



43. Sandstone, 15 to 20 feet. 



42. Drab clay and iron ore. 



41. Coal, (So. 10,) no develoi)ment. 



40. Yellowish drab clay, 4 feet. 



39. Sandstone, 20 feet. 



38. Drab clay full of the finest quality of iron ore, 15 feet. 



37. Thin layer of saudstone. 



30. Coal, (No. 9,) nearly vertical, where it has been worked, 12 feet. 

 35. Arenaceous clay, 2 feet. 



34. Drab clay, 3 feet. 



33. Sandstone, 5 feet; then a heavy seam of iron ore; then 3 feet of 

 drab clay ; then 5 feet sandstone. 

 32. Coal, (No. 8,) 4 feet. 



31. Drab clay. 



30. Sandstone, 25 to 40 feet. 

 29. Drab clay, 6 feet. 

 28. Coal, (No. 7,) 6 feet. 

 27. Drab clay, 5 feet. 



26. 7 • C Sandstone with a seam of clay, 12 to 18 inches, intercalated, 

 25 feet. 



Drab clay, 4 feet. 



Coal, (No. 6,) in two seams, 4i^ feet. 

 23. 3 ^ I Drab clay, 3 to 4 feet. 

 .22. Yellowish, tine-grained sandstone in thin loose layers, with plants, 

 5 to 10 feet. 



21. ) §j ( Drab clay, excellent iron ore. ) 



20. > a i ^0^1' (^o- ^?) ^ fee*- ( ^^ feet. . 



19. ) Q ( Drab clay. ) 



I- 

 24. I B 



