iQ2o] TAYLOR— SUCCESSION OF MOSSES 475 



pioneer xerophytic species soon give way to the more mesophytic 

 ones. From the edge there is usually a rather abrupt slope upward 

 for a few rods, which is thickly wooded, in most cases with oaks 

 sparsely sprinkled with red cedar, and here and there a white pine. 

 The undergrowth is decidedly mesophytic, and on the rocks are 

 the same mosses already given for the other moist shady habitats. 

 Immediately beyond the strip of wooded land are cultivated 

 fields. 



In comparing the sparse moss flora on rocks of the Chicago 

 region with the very luxuriant display along Carroll Creek, where 

 general climatic conditions must differ only slightly, one at once 

 begins to search for the cause of the variation. While the rock 

 exposures around Chicago are not extensive, they are sufficient 

 to serve as a basis of comparison. The rock in both cases is 

 dolomitic limestone, not differing enough in structure to be an 

 important factor. The only outcrop which is near enough to 

 Lake Michigan to be affected by the greater humidity is that of 

 Stony Island, and that is, if anything, more barren than are the 

 other regions. The cliffs and ravines at Lemont are not close to 

 the stream as are those at Mount Carroll, but are on what was 

 probably the river bluff at some past period when the stream 

 contained much more water than at present, in all probability 

 when the Des Plaines River was the outlet of the old Lake Chicago. 

 Now the cliffs are not near any body of water, and in the ravines 

 are only small streams which are nearly dry a part of the year. 

 The stone quarry at Thornton is being worked by a cement factory, 

 so that the exposure, w r ith the exception of the rocks along the 

 top, is too recent to afford any information. The amount of 

 moisture which could come from the pool of water in the bottom of 

 the quarry cannot be great enough to affect the flora on the 

 horizontal rock surfaces above. The quarry at Lemont has been 

 abandoned for some time, and much of the bottom is overgrown 

 with weeds and grasses. The pools of water in the depressions 

 may add slightly to the humidity of the air in the immediate 

 vicinity; while the vegetation growing up from below and that 

 overhanging from the upper edge of the rock undoubtedly adds 

 to shade and contributes to a lower rate of evaporation. The 



