1884. | Opinions upon Clay Stones and Concretions. 883 
action has been assumed to indicate a cohesive and attractive 
property in matter when finely divided, and when its particles 
possess some or considerable freedom of motion, whereby mole- 
cules of the same sort gather together in bunches or globes. 
‘sometimes coat over coat, the whole enlarging until the limit of 
cohesive action around that center has been reached, or the ex- 
panding circumference of one concretion meets and impinges 
upon its growing neighbors in a mass affected throughout by this 
“toward-a-center” movement. This is a partial confession of 
ignorance as to what the essential nature of the process is. We 
are led to believe from the analogy in all these cases enumerated 
above, that the action is the same, and perhaps it is, but modified 
by varying conditions and the constitution of the substance 
influenced. 
It is doubted whether basaltic columns can ascribe their forma- 
tion to concretionary action, and it is plainly stated that geodes 
arise from the entrance of saturated solutions from without, 
through fissures, into the cavities formed within clay or other 
nodules, by internal shrinkage, of which process the familiar hol- 
low iron ore balls are a good illustration. 
The authorities are not inclined to throw much light upon this 
curious phenomenon, regarding it as an ultimate fact in nature. 
Dana gives no explanation of this process (Manual, 1879, pp. 85, 
86, 87, 88) but illustrates it in various figures. In the Manual, 
1875, he speaks of concretions “ having the form of, or containing 
spheroidal concretions; some varieties are also called globulifer- 
ous when the concretions are isolated globules and evenly dis- 
tributed through the texture of a rock; others are oolitic when 
made of an aggregation of minute concretions not larger than 
the roe of a fish.” He speaks of one example as “a crystalline 
rock with spherical concretions imbedded in its mass and not 
separable from it * .* * each layer (of the three forming 
each concretion) consisting of different minerals, for example, 
garnets the center, feldspar the middle layer and mica the outer, 
and all making a solid mass. The constitution of such con- 
Cretions is very various. In rocks containing feldspar they 
Usually consist largely of feldspar and sometimes of feldspar 
alone or of feldspar with some quartz. The concretions in pitch- 
Stone and pearlstone (called spherulites) are almost purely feld- 
spathic, and often separate easily from the rock.” He figures 
VOL XVIII.—No, IX. 
