172 THE PEINCIPLES INVOLVED IN EOCK-WEATHEEING 



meteoric waters that still another authority^ would attribute 

 the slight thickness and nodular condition of many beds of 

 Palaeozoic limestone. He argues that originally thick-bedded 

 limestones have, during the ages subsequent to their formation 

 and uplifting, become so impoverished through the dissolving 

 out and carrying away in solution of the lime carbonate, as to 

 have been quite obliterated, or reduced to mere nodular bands, 

 and given rise to important pateontological breaks in the geo- 

 logical record. Other than organic acids may locally exert a 

 potent influence. Thus Eobert Bell has described the dolomitic 

 limestones underlying the waters along Grand Manitou Island, 

 the Indian peninsula, and adjacent portions of Lake Huron an,d 

 the Georgian Bay, as pitted and honeycombed in a very pecu- 

 liar and striking manner. This corrosion, it is believed, is 

 produced through the solvent action of sulphuric acid in the 

 water, the acid itself arising from the decomposition of the sul- 

 phides of iron, pyrites and pyrrhotite, which exist in great 

 quantities in the Huronian rocks to the northward.^ 



1 F. Rutley, The Dwindling and Disappearance of Limestones, Quar. Jour. 

 Geol. Soc. of London, August, 1893. 



2 Bull. Geol. Soe. of America, Vol. VI, pp. 47-304. 



Messrs. C W. Hayes and M. B. Campbell, of the United States Geological 

 Survey, have reported some remarkable examples of corroded quartz pebbles 

 which should be mentioned here, although a satisfactory explanation for 

 the phenomenon has not yet been given. 



Dr. Hayes, in a personal memorandum to the writer, describes the occur- 

 rence as follows: 



'^At three rather widely separated points in the South, conglomerates 

 have been observed in which the projecting portions of the pebbles have 

 been etched or partly dissolved. 



' ' The first, observed by Mr. Campbell, is at Nuttall, West Virginia. The 

 conglomerate in question, which belongs to the coal measures, is composed 

 of rather coarse quartz sand with slightly yellowish cement, in which are 

 embedded well-worn pebbles of white vein quartz. The latter vary in size 

 up to three-quarters of an inch in diameter, and are somewhat irregularly 

 distributed. Ordinarily the pebbles, wholly unaltered, weather out by the 

 chemical or mechanical disintegration of the sandy matrix. In the case 

 observed, however, where the conglomerate received the drip from an over- 

 hanging cliff, the projecting portions of the pebbles are deeply pitted evi- 

 dently by solution. Mechanical wear is precluded by the form of the re- 

 sulting surface, which is not smooth like the portions of the pebble still pro- 

 tected by the matrix, but is rough and irregular. The outer portion of the 

 pebbles is evidently less easily ajBtected by the solvent than the interior, 

 and forms a sharp rim about the regular cavities hollowed out within. In 

 some cases a third of the pebble has thus been removed. The surface of 



