98 



LARAMIE FLORA OF THE DENVER BASIN. 



The presence of very considerable veins of 

 coal, which occur mainly in the lower portion 

 of the formation, undoubtedly calls for wide- 

 spread and long-continued swamp or marsh 

 conditions. David White," in speaking of the 

 physiographic conditions attending the forma- 

 tion of coal, said: 



The enormous horizontal extent of many of the coal 

 g oups as indicated by the remnants now found in basins 

 detached as the result of folding and erosion, the demon- 

 strated continuity of some of the individual coal beds over 

 areas of amazing size, the high degree of parallelism of the 

 beds, and the recognition that the coal beds were laid down 

 beneath a water cover join in predicating the existence, at 

 the time the coal was being formed, of vast swamps and 

 broad but shallow lagoonal areas subject to subsidence at a 

 generally slow rate. 



The above is known as the autocthonous 

 (formed in place) hypothesis of the origin of 

 coal. It is but fair to state, however, that the 

 allocthonous (transported) hypothesis, which 

 argues that much of the coal may have been 

 formed by floating and sedimentation at the 

 bottom of open lakes or lagoons, has still an 

 occasional advocate. In a recent article on 

 "The mode of origin of coal" E. C. Jeffrey'" 

 set forth the results of the microscopic exami- 

 nation of great numbers of coals from all parts 

 of the world and from horizons ranging from 

 Devonian to Tertiary. He held that he is able 

 to distinguish the relatively few types that ap- 

 pear to have been formed under peat-swamp 

 conditions and asserted that the vast majority 

 give evidence of deposition in open water. 

 ''Coal is not a compost heap but a sedimentary 

 deposit," he concluded. Be this as it may, it is 

 not within the scope of the present paper fur- 

 ther to discuss the origin of coal. 



Concerning the interpretation of the climates 

 of the coal-forming periods. White said: 



It will be seen that duringthe times of deposition of moat 

 of the p:incipal coal groups the climate has been charac- 

 terized by (1) general mildness of temperature, approach- 

 ing in most cases tropical or subtropical conditions; (2) con- 

 spicuous eriuability or approximation to uniformity of 

 climatic conditions, which, with few exceptions, appear 

 to have lacked cold winters or severe frosts; (3) a generally 

 high humiditv, the rainfall being from moderately heavy 

 to very heavy and fairly well distributed during the yea-, 

 though in many cases there is evidence of the occurrence 

 of dry periods, which, however, seem ordinarily to have 

 been comparatively short and not severe; (4) an amazingly 



• White, David, and Thies-cn, Roinhardt, The origin of coal: Bur. 

 Minc< Bull. :«. p. .53. ISl.l. 

 "Jour. Geology, vol. 2), pp. 218-23% 1915, 



wide geographical distribution of these genial and equal le 

 climates. 



To repeat, then — the abundant presence of 

 coal in the Laramie of the Denver Basin 

 offers in itself sufficient proof of the existence 

 of long-continued, relatively uniform swamp 

 or marsh conditions. According to currently 

 accepted theories a larger percentage of all 

 coals were formed by accumulations of vege- 

 table matter under conditions similar to those 

 existing in a modern peat swamp. The types, 

 of vegetation that contributed to the for- 

 mation of the coals depend of course upon the 

 geologic period during which they were laid 

 down; but whatever the age, the process is 

 believed to have been essentially identical 

 throughout. 



We may now examine the plants found in 

 the flora under discussion to ascertain if 

 possible which of its members appear to 

 demand a habitat that would make it a 

 potential contribution to or at home in such a 

 peat-producing environment. The sedgelikc 

 plants described as Ci/peracites might very 

 well have found a home in such a swamp, and 

 the tall, canelike grass Phragmites is notably 

 such a swamp-loving plant. HinUax — or at 

 least the form to which the Laramie species 

 seems to be related — is essentiall}" a denizen of 

 swamps and low places, where it often forms 

 dense tangles. Equally at home in or about a 

 swamp, or along adjacent watercourses, would 

 be found the waxberries ( Myrica), willows, and 

 jjopjais, while the shallow, (|uiet waters 

 furnished a congenial surrounding for the 

 floating leaves of Nelamho. Among the ferns 

 there are several, such as (hiodea, Ly(fodiitm, 

 and Anemia, that might well have found a 

 home in or near swampy or low ground, wiiiie 

 the others, including Asplenium, Dryopteris, 

 etc., demand shade and moisture such as is 

 afforded in or adjacent to low ground. 



If the data have been correctly interpreted, 

 tlie Moraceae formed the most conspicuous 

 group in this flora, comprising two genera and 

 nearly 25 species. Of (hesc Artocarpu^ is 

 represented by two very weli-niMrked forms. 

 The members of tliis genus at the present 

 time demand a warm, moist habitat such as 

 might be afforded by tlic swamp we are 

 predicating. In the same way the several 

 species of Ficiis call for like conditions. This 

 great swamp may well have harbored also the 



