66 QUICKSILVEIJ DKI'OSITS OF THE PACIFIC SLOPE. 



the action of some substance embedded in the rock. The spherical form 

 which they tend to assume and to which they often closely approximate 

 indicates that this substance either exists or once existed at the center, and 

 this simple inference is confirmed by the fact that the concretions are often 

 separable into spherical shells, indicating a change in composition related 

 to the distance from the center. Such concretions are common, for exam- 

 ple, in the Chico beds near Lower Lake. So far as my observation goes, 

 the central substance has utterly disappeared in a great majority of cases. 

 In three or four instances I have found fossils at the centers of these con- 

 cretions, but I have broken open great numbers of them without finding 

 any visible diff"erence in the substance at the center and elsewhere. Such 

 was the case in the specimen investigated, and chemical tests also failed to 

 detect any foreign substance. Thus, while fossils are occasionally found 

 both in California and elsewhere at the centers of concretions in sandstone, 

 such cases are so rare that one would by no means be justified from obser- 

 vation in ascribing the concretions to the action of organic matter, nor am 

 I aware that it has ever been shown how organic matter could efi"ect such a 

 result. 



Nucleus possibly organic. — Tiie aualysls glvcn above appears to me capable 

 of interpretation in a manner calculated to throw light on the nature of the 

 central substance. The cement of the concretion contains one-fourth of 1 

 per cent, of phosphoric acid, most of which nuist have come from the cen- 

 tral substance, for the cement of the sandstone away from concretions is 

 almost pure calcite. Taking the size of the concretions into account this 

 indicates the existence of much phosphorus in the central substance. The 

 possibly organic nature of the substance in question is thus strongly sug- 

 gested. But is there any way in which an organic substance could give 

 rise to the formation of hydrous ferric silicate? I think there is. 



It is well known that during the decomposition of organic matter 

 various acids are formed, and that among tliem the group of humus acids 

 frequently occurs. Tlicse acids, or some of tlieni, dissolve magnetite, and 

 in this way sands underlying vegetable soils are frequently bleached.' 



'Rolli: Allji. iin.l chciii. Ge.>l., vol. 1, ii. SDC: ami A. A. Jiilieu : Oil the geological action of tiii' 

 humus acids, Pioc. Aui. A>so(;. A<lv. Si-i., vol. 28, IS/il, iiji. :U1-410. 



