TO JUNCTION OF GRAND AND GREEN RIVERS. 77 



places throughout the year, while from the Sandia and Santa Fe Mountains — over 

 12,()()() feet in heighl — it entirely disappears in midsummer. Only a small portion ol 

 the range came under our observation ; but enough was seen to show that its struc- 

 ture was essentially the same with that of the other chains belonging to the same 

 scries which we were able to examine more fully. The southern extremity is mostly 

 eruptive in character, consisting of basaltic trap, porphyry, and trachyte, some of the 

 latter containing crystals of feldspar of large size. As usual, the summits composed of 

 these materials present very varied and picturesque outlines. In the opposite chain 

 ot the Sierra del Navajo arc; some castellated summits, which, as seen through a 

 powerful glass, presented an appearance exceedingly imposing and beautiful. Among 

 these are precipices, ornamented with imitations of columns, arches, and pilasters, 

 which form some of the grandest specimens of nature's Gothic architecture 1 have 

 ever beheld. When viewed from some nearer point they must he even awful in their 

 Bublimity. 



The Cretaceous strata at the base of the San Juan .Mountains are highly fossil- 

 liferous, and yielded me a large number of species. The ferruginous, sandy lime- 

 stones and shales near the base of the middle division of the Cretaceous series, where 

 exposed JUSI north of the PagOSa, are almost made up of the shells of Ostfea, Ii/occra- 

 mus : Ammonites, &C. This is the horizon from which tin; oysters, gryphajas, fish- 

 teeth, Sir., were collected in such abundance on the Canadian, at Galisteo, at the ford 

 of the Chania, and various Other points on our route. 'These fossils are limited to the 

 first hundred feet of the shale-heds ahove the Lower ( Yetaceous sandstones ; the most 

 abundant at the Pagosa are Ostrea lugubris, 0. wniformis, .Meek, n. sp., Tnoceramus prob- 

 l&maticus, and /. frayUis I also found here an Interesting series of the teeth of 

 Ptychodus Whijpplei, described at length in the chapter on Paleontology. Above the 



beds in which these fossils are found, the black bituminous shales contain large, gener- 

 ally broken, shells of Nautilus and Tnocero/mus, thickly set with Ostirea congesta. 



All the interval between the PagOSa and lvio I'iedra is occupied by CretaceOUS 



rocks. Over large surfaces the Lower Cretaceous sandstones are laid bare, generally 

 much disturbed, and somewhat changed. The only fossils discovered were indistinct 

 vegetable impressions, among which the most conspicuous is a tuberculated fucoid 

 (Halymenites) -common in the Cretaceous sandstones of all this region. Here, as at the 

 Tierra Maria, these rocks are cul by joints into blocks of nearly uniform size. Of these 

 joints the most strongly marked have a bearing approximately north and south; the 

 others cross these nearly at right angles, [n the great number of localities in New 

 Mexico where I observed jointings of these rocks, I found them to have similar direc- 

 tions, and to be in no case influenced by local disturbances, nor having any obvious 

 connection with the prevailing dip. r I uis rule is of 80 general application that 1 have 



been driven to refer the jointing of all the rocks of this region to one great common 

 cause. This has seemed to me probably magnetic, but may possibly be connected 

 with the action of the forces which have sriven to all the great lines of elevation a 

 general north and south trend. 



On the Nutria Frances, a small tributary of the Rio Piedra, the black shales of 

 the Middle Cretaceous are exposed in various places. They here contain large Itto- 



