816 



SCIENCE. 



[N. S. Vol. XXIII. No. 595. 



able that the organic residue represents as 

 little as one twelfth of the original volume. 



The Ordovician, like the Carboniferous 

 gelosic algas, appear to have exercised an at- 

 tractive or elective influence on bituminous 

 compounds, particularly those of illuminant 

 values, and to have consequently been perma- 

 nently somewhat enriched. Portions of their 

 hydrocarbon contents have doubtless been lost 

 at various periods, and the great shrinkage of 

 the shale which caused the collapse of the 

 overlying limestone strata probably marked 

 ■one, perhaps the first, of these periods of 

 hydrocarbon reduction after their original 

 sedimentation. Presvunably accelerated loss 

 ■occurred at all times of rock folding in the 

 region.- Such occasions might be favorable 

 for the deeper zinc deposition. 



The gelosic bodies found in the Ordovician 

 oil shales of the zinc region not only explain 

 the localization and immediate source of the 

 hydrocarbons which have affected the deposi- 

 tion of the zinc ores in this part of the Missis- 

 sippi Valley, but they also ofier an hypothesis 

 which it is believed will prove satisfactory, 

 though subject to conditional modifications, 

 in explanation of the origin of the oil and gas 

 in rocks of Paleozoic age in other basins. 

 There can be no doubt that similar organisms 

 swarmed in other parts of the same sea or 

 other seas, and that, whether or not they have 

 been recognizably preserved as in the light 

 chocolate shales near Plattville, they have con- 

 tributed enormously to the hydrocarbon con- 

 tents of their respective formations. 



At the 180th meeting on April 25, the fol- 

 lowing papers were read: 



A Map and Crbss-seciions of the Downtown 

 District of Leadville: Mr. S. P. Emmons. 

 The Downtown district includes the streets 

 and buildings of the city. Its surface is cov- 

 ered by 200 to 500 feet of glacial material, 

 forming a gently sloping tableland which ex- 

 tends to the Arkansas Valley and must be 

 passed through by mine shafts before they 

 can reach the underlying rock in place. Very 

 large bodies of ore, mainly in oxidized form, 

 have been and are still being discovered in the 



underlying rocks, and it is to aid in their 

 development that this map is being prepared 

 in advance of the general map of the district. 



In the early surveys of the district, made 

 in 1880-81, Mr. Emmons distinguished two 

 divisions in the gravelly material that covers 

 the rock in place : the lower, which is stratified 

 and consists mainly of fine-grained sands and 

 marls, was called 'lake beds,' while the upper 

 division, which consists entirely of unstrati- 

 fied boulders and clay, was called ' wash.' 

 The former beds were assumed to have been 

 deposited in a glacial lake ponded back in the 

 upper Arkansas Valley by the glaciers which 

 issued from the lake fork of the Sawatch 

 Mountains near Twin Lakes during the first 

 glacial period. In the terms of modern physi- 

 ography, these deposits would be for the most 

 part more properly classed as glacial-fluvial 

 beds, as has been claimed for the whole by 

 glacialists who have recently been studying 

 the Twin Lakes region; but Mr. Emmons still 

 inclines to the belief that a lake covered a 

 part, at least, of the upper Arkansas Valley in 

 which the finer materials, issuing from be- 

 neath the earlier glaciers, were deposited. 

 The faulting of these beds, shown in the 

 recent mine workings, proves a certain amount 

 of uplift in the region back of Leadville since 

 the glacial period. 



In the original survey it was shown that the 

 slopes of the Mosquito Range, back of Lead- 

 ville, consist of fault blocks successively up- 

 lifted toward the east along north and south 

 striking faults, in which the sedimentary beds 

 lie in shallow synclines, the faults themselves 

 following the steeper limb of the anticline. 

 Recent underground developments have shown 

 that, while the main fault planes were cor- 

 rectly located, their displacement was often 

 distributed on several planes, so that the beds 

 to the west of each fault zone descended not 

 with the even slope of a syncline, but in a 

 series of steps. Depths below the surface of 

 the bottom of the basins, as determined on 

 the syncline theory, were in general correctly 

 given in the successive cross-sections of the 

 original map. In the case of the block repre- 

 sented by the down-town area, however, no 

 shafts had penetrated the covering of glacial 



