110 EXPLORING EXPEDITION FKOM SANTA Ffi 



stones. Whether all this great group belongs, like its base, to the Cretaceous system, 

 we have at present no means of determining ; the only fossils found being such as do 

 not suffice to solve the problem. These are, so far as observed, silicified trunks of large 

 coniferous trees, and abundant, though rather obscure, impressions of angiospermous 

 leaves of .species different from any found elsewhere in the Tertiary or Cretaceous 

 strata. 



The character of the marls of this series is very well given in my notes upon the 

 lowest bed exposed at Camp 43, a few miles below the mouth of Canon Largo. These 

 notes are as follows: 



"The hills which bound the valley opposite camp are about 250 feet high, com- 

 posed of greenish-gray and yellowish- white marls, generally very soft, but occasionally 

 containing lenticular masses of coarse sandstone and conglomerate. Dispersed through 

 the marls are many spherical concretions of ferruginous sandstone which decompose 

 on exposure; plates of selenite, in places covering the slopes, and masses of water- worn, 

 silicified, carbonized, calcified, or gypsumized wood. All the beds contain much 

 saline matter (mostly sulphates) which effloresces on the exposed surfaces. The pre- 

 vailing color of these marls is whitish, and in their general aspect the hills which 

 they form closely resemble the chalk-hills of Southern England." 



At Camp 44, 1 made the following entry in my journal : 



"To-day we reached the mouth of the Canon Largo, and the crossing of the San 

 Juan. The valley is here, as below, moderately fertile, bounded by rounded hills of 

 chalky strata, with terraces of gravel near the river ; all covered with good grass. The 

 soil of the bottom-lands is alkaline, often covered with salt-bush (Sarcobatus), but sus- 

 taining extensive groves of cotton wood and thickets of buffalo-berry ; the latter loaded 

 with fruit. In geology little change is noticeable; the dip of the rocks continues east- 

 erly, and we have the chalky marls mentioned in yesterday's notes, succeeded by a 

 stratum some 50 feet in thickness of yellowish, soft, oolitic, coarse-grained, calcareous 

 sandstone, so friable as to be scarcely transportable, but very massive, and standing 

 in perpendicular faces in the cliffs. In this stratum is a band of gray clay-shale, in 

 which I find abundant impressions of leaves, but have been unable to obtain any 

 specimens sufficiently perfect for determination. 



"Above this sandstone is a bed of greenish- white and purple marl, weathering like 

 the marls of the Trias, and closely resembling them in all things. Above the marl is 

 another bed of sandstone similar to that below ; then another bed of marl, &c." 



At Camp 45, my journal says: "Canon Largo is bounded by high and precipitous 

 walls on either side, composed of soft yellow sandstones, and white and variegated 

 marls. The cliffs are not absolutely perpendicular, but ascend in grades; the sand- 

 stone forming vertical faces, the marl, slopes." 



"The alternations in this series present very little variety; the sandstones, some 

 ten in number, being very like each other ; the marls, of about equal force, showing 

 no marked differences. They often form rounded but furrowed hills, of which the 

 surfaces are whitened by a saline efflorescence or are covered with fragments of ferru- 

 ginous concretions and fossil wood. This silicified wood is very abundant and seems 

 to be all coniferous." 



