Maryland Geological Survey 175 



varied that it could furnish almost any of the more usual rock-forming 

 minerals ; the other is a fact that is, perhaps on account of its unwelcome 

 character, all too generally ignored in work of this kind, namely that 

 the older sedimentary rocks — limestones, shales, or sandstones — contain 

 heavy minerals just as do the rocks being studied, and that a region of 

 sedimentary rocks is not going to yield, at least at a distance, fragments 

 of limestone and shale, but rather the mineral grains that were included 

 in the limestone and shale. Thus the problem is seen to be a very com- 

 plicated one, in which only the most general results are readily obtained. 

 If this side of the work is to be developed it will probably be necessary 

 either to find unusual minerals and trace them, or else to differentiate by a 

 close mineralogic study varieties of common minerals, such as feldspars, 

 augites, hornblendes, or even quartz, as Mackie has done, 1 and then trace 

 down to its source the particular variety thus identified. This requires, 

 however, close study not only of the sediments but also of the rocks from 

 which their minerals may have been derived, and this becomes a long 

 and arduous problem. Without such work the study of mineral grains in 

 sedimentary rocks does not, in most cases, yield much of value. 



It will have been noted that in all the sediments studied the coarser sizes 

 of sand had a glossy pitted surface which seemed plainly to indicate solu- 

 tion of the grains after deposition. This phenomenon appeared so gen- 

 eral that it cannot be connected with the particular composition of the 

 bed. Evidently the ordinary circulating ground water is the agent. The 

 chemistry of the process is not understood, though humus waters are 

 supposed to be particularly effective. According to the more recent 

 theories, which deny the existence of humus acids, this is probably due to 

 the carbonic acid. 



More limited in its observed occurrence in these samples is the deposi- 

 tion within the sediment of quartz from solution. The evidence for this 

 appeared most convincing in sample 13, but associated with deposi- 

 tion of silica there is here to an unusually pronounced degree the same 

 solution of silica as noted on the quartz grains in most of the other 



1 Mackie, Wm, The sands and sandstones of E. Moray. Trans. Geol. Soc. 

 Bdinb. vol. 7, 1896, pp. 148-172. 



