282 Barr ell— Physical Effects of Contact Metamorph 



ism. 



sary for the disintegration of a rock, and secondly that found 

 in material which has suffered long transportation and tritura- 

 tion, such as the residual clay from a limestone. 



Amount of Decomposition in situ. — Merrill has shown in 

 the articles to which reference has been given that in the 

 instances studied by him the transition from fresh rock to soil 

 in situ has been brought about with very little change in ulti- 

 mate chemical composition — an addition (in the case of the 

 Washington granite) of some 3*5 per cent of water, a change 

 of the ferrous iron to ferric, doubtless more or less hydrated, 

 and a slight decrease in the total amounts of silica, lime, 

 potash and soda being the more conspicuous features. " It is 

 evident that here the chief alteration in the conversion of the 

 barren rock into arable soil is physical, attended probably with 

 a partial change in the mode of combination of the various 

 elements."* 



In his investigation upon the changes taking place from 

 fresh rock to soil in situ a separate analysis was made of the 

 impalpable mud or silt derived from the soil and remaining 

 some time in suspension in water. Of this, representing 4 

 per cent of the total disintegrated material, only 39' 7 per cent 

 was soluble in dilute hydrochloric acid, and a considerable 

 proportion of the residue, as indicated by the high percentages 

 of alkalies and lime, still consisted of unaltered soda-lime and 

 potash feldspars, the iron and magnesia alone having been 

 largely removed. This power of the alkaline feldspars to 

 strongly resist decomposition, even when finely divided, is to 

 be noted, since the study of thin sections under the microscope 

 shows that one of the common earliest results of rock decay is 

 the filling of the feldspars with sericite Hakes or kaolin dust. 

 From this fact alone it would be judged that the feldspars 

 upon disintegration to an impalpable mud would be readily 

 converted to kaolin, the final product of alteration ; yet such 

 proves not to be the case, a considerable proportion of them 

 remaining unaltered and therefore suffering no loss of com- 

 ponents when subjected to metamorphism. 



Decomposition in Transportation and Sedimentation. — In 

 the undisturbed soil of an igneous rock, however, the fullest 

 opportunities have not yet been given for hydration, oxidation 

 and the formation of carbonates. To test the limit to which 

 such actions proceed under favorable circumstances, instances 

 should be studied where transportation and sedimentation have 

 given the fullest opportunities for complete alteration. 



The residual clay from a limestone meets the requisite 

 conditions, having been finely comminuted and subjected for a 

 long period of time to such agencies before settling to the 



* Bull. Geol. Soc. America, vol. vi, p. 324. 



