G8 QUICKSILVER DEPOSITS OF TDK TACIFIC SLOPE. 



very great. It is clear that the formation of concretions is clue to the pres- 

 ence of small masses of some foreijjn substance in tlie sandstone. Were 

 this substance composed of any other elements than carbon, hydrogeu, 

 nitrogen, and phosphorus, such elements would almost inevitably appear as 

 components of the concretion. It thus appears to me nearly certain that 

 the concretions are due to the action of decomposition-products arising 

 from organic matter. 



It is evident that the formation of concretions by means of organic 

 matter, as sketched above, is a result which will take place only under some- 

 what special conditions. If sufficient organic matter exists in a rock, indu- 

 ration of the entire mass may occur. If the huniic components are washed 

 through the rock without being allowed to decompose, the rock will be 

 bleached. Both of these last cases arc considered by Professor Julien in 

 the paper referred to above, but he does not particularly discuss the subject 

 of concretions 



NODULES RESlTLTING FROM EXTERNAL ATTACK. 



Cases to be discussed. — Bcsides the concrctions discussed above, rounded nod- 

 ules are found in many- decomposing rocks. In the present memoir such 

 occurrences will be noted in the basalts of the Sulphur ]3ank mine and in 

 the partially serpentinized rocks, especially near Knoxville, Napa County. 

 They are also well known to occur in some decomposed granites and in an- 

 desites, those for instance near the Comstock lode. The principles on which 

 they are formed are extremely simple, but, so far as I know, they have never 

 been stated, and a lack of knowledge of them has often led to erroneous 

 assumptions of a mysterious ball structure in the rocks which favors such 

 decomposition. As will be seen below, pebbles in brooks and on beaches, 

 as well as grains of sand, are rounded in a manner closely analogous. 



Deduction of relations. — Suppose a splicrc of any homogeneous substance, 

 into which liquids can penetrate a small but finite distauce, and let this dis- 

 tance be assumed as the unit of length. Then, if r is the radius of the 

 sphere, the volume of the solid which can he permeated by a liquid acting 

 on the exterior is a spherical shell, tlie content of whicli is : 

 4 4 4 



