64 QUICKSILVER DEPOSITS OP THE PACIFIC SLOPE. 



ferromagnesian silicates appears to be identical with that which results 

 from the similar minerals of the recrystallized rocks. The nature of this 

 chlorite will necessarily be discussed in connection with these rocks. Ser- 

 pentine has not been identified with certainty among the results of weath- 

 ering in these sandstones. It is possible, however, that it forms a very 

 subordinate product of this process. The decomposition of the feldspars 

 calls for no special comment, excepting that there is no considerable quan- 

 tity cf well marked kaolin. 



CONCRETIONS. 



Analysis of an example. — Ouo of tlie most iuterestiug cliauges which take 

 place in the sandstones is the formation of concretions. These are very 

 common in the Chico-Tt^on and Miocene groups. That they really repre- 

 sent changes of composition within the rock mass is certain, for they often 

 develop into a symmetrical, spheroidal shape, without disturbance of the 

 stratification, which the formation of conci-etions does not wholly obliterate. 

 That these concretions could not gradually be built up during sedimenta- 

 tion is certain. They are usually much harder than the surrounding rock, 

 darker, and of a redder color. In a great majority of cases no nucleus can 

 be found at the center. Under the microscope the chief peculiarity of 

 these concretions was found to consist in a brown cement between the 

 clastic fragments of granitic origin. This cement does not effervesce with 

 acid and is so unusual in character as to call for investigation. 



A concretion from the Chico beds of New Idria (No. 53) was selected 

 for examination. One sample of the pulverized rock was treated with cold, 

 dilute chlorhydric acid (1:10) and the resulting solution analyzed. Another 

 sample was digested with stronger acid (1:1), at first at ordinary tempera- 

 tures and then for twenty-four hours ou the water-bath. The solution 

 formed was analyzed and the residue was treated with a hot solution of 

 sodic carbonate to extract soluble silica. Special determinations were 

 made of carbonic acid, ferric oxide, etc. The following talde shows the 

 pei'centages soluble in weak acid and in strong acid separately. 



