hatden.] GEOLOGY LIGNITIC GROUP IN COLORADO. 29 



ion, which are supposed to be of Cretaceous age, but which might very 

 properly be called beds of passage from well-marked Cretaceous strata to 

 those containiug coal and vegetable remains. We have heretofore de- 

 scribed the Upper Cretaceous beds as of strictly marine origin; that the 

 sediments were deposited in a broad and, at least, moderately deep sea. 

 As long as we find that these physical conditions prevailed, we observe a 

 greater or less abundance of fossils of strictly marine forms, as Ammon- 

 ites, Baculites, Inoceramus, etc.; but even when no break can be found in 

 the sequence of the beds, indicating a lapse of time in the deposition of 

 the sediments, we discover that the physical conditions gradually change 

 until there is a complete extinction of all marine forms of life. We find 

 here on the Arkansas River a full development of the Upper Cretaceous 

 formations Nos. 4 and 5, with theirpeculiar fossils. We also observe that 

 the materials of the upper portion of No. 5 pass gradually from a dark- 

 yellow clay to a rusty-yellow sand, and above this, 200 to 300 feet, of a 

 sort of irregular thin layers of mud-like material, with curious concre- 

 tions of sandstone. In this* group of strata, which may be called transi- 

 tional, not a fossil has yet been found to prove the age beyond a doubt. 

 Resting on this irregular group of mud-strata is a bed of sandstone of 

 very variable thickness as well as structure. Sometimes it is not more 

 than 50 feet thick, and then again it is 300 to 400 feet thick. It is full 

 of rounded concretionary masses, and shows very clearly that its sedi- 

 ments were deposited in shallow and very turbulent waters. This sand- 

 stone passes up into clay, and on this clay rests a bed of coal. In the 

 bed of sandstone below the coal, the peculiar vegetation of the Lignitic 

 group is found in considerable abundance, and, therefore, this may mark 

 the lowest horizon of this group. Now, whenever, in any part of the 

 country, invertebrate remains of any kind are found above this bed of 

 sandstone, they are invariably brackish or fresh water in their charac- 

 ter; and whenever any of these fossil shells are observed below this sand- 

 stone, they are always strictly marine. We have in the vicinity of these 

 coal-mines the details of structure, which we have briefly described 

 above, most clearly shown. Now the question arises, what stress shall 

 be laid on these remarkable physical changes 1 Would not this form an 

 excellent line of separation between two great periods in geological 

 time? Are not these changes sufficient to indicate clearly that these 

 are probably the beds of passage or transition between the Cretaceous 

 and the Tertiary epochs ? We find also a complete change in the vege- 

 table as well as animal life. We are not aware that any of the verte- 

 brate remains, which have been regarded by Cope and Marsh as proving 

 the Lignitic group to be of Cretaceous age, have ever been found mingled 

 with any other forms of life of strictly marine origin. So far, all the 

 vertebrate fossils have been discovered in the Lignitic group. It seems 

 therefore that not a single species of vegetable or animal life survived 

 the physical changes which were introduced during the time of the depo- 

 sition of the transition group. Now, if we have shown this state of 

 affairs in regard to the Carton group, we may connect this group easily 

 with the Raton Hills group to the southward, and the Monument Creek 

 group far to the northward near Colorado Springs. 



In passing northward, we see no more of the Lignitic group, so far as 

 we have examined, until we reach Colorado Springs, a distance of thirty 

 miles in a straight line. Here it is exposed in the form of an irregular 

 bluff ridge, running down from the base of the mountains a little south- 

 east, beyond the limit of our explorations up to this time. In section 2, 

 we see in the foreground, at either end, the form of the sandstone bluffs, 

 which appear to be remnants of a far more extended group of strata. 



