1884. ] Opinions upon Clay Stones and Concretions. 885 
this process as concretionary action. There is no objection to be 
raised to this phrase, and it or some similar term may be safely 
and conveniently used to express the fact that certain matters 
have been separated out of the body of the rock and collected 
together in balls, provided always we bear carefully in mind that 
by giving the process a name we do not get any nearer to under- 
standing the manner in which the result has been brought about. 
If any one asks us what made the nodules, we may, if we like, 
say concretionary action; but if the awkward question is put, 
What is concretionary action? we should be somewhat puzzled 
for an answer. We know that one of the ingredients of a mix- 
ture has been extracted from the surroundings and gathered into 
lumps; how exactly this was done we do not know. The term, 
in fact, is only a way of stating our ignorance, and, unless due 
precaution be taken, a somewhat dangerous way, because to cer- 
tain minds it looks like an explanation.” 
This remark of Professor Green echoes the German couplet : 
“Denn eben wo Begriffe fehlen 
Da stellt ein Wort zur rechten Zeit sich ein.” 
Archibald Geikie says (Text Book of Geology): “ Concretion- 
ary, containing or consisting of mineral matter which has been 
collected either from the surrounding rock, or from without, 
round some center so as to form a nodule or irregularly shaped 
lump. This aggregation of material is of frequent occurrence 
among water-formed rocks where it may be often observed to 
have taken place round some organic center such as leaves, cones, 
shell-fish remains or other relics of plants or animals. Among 
the most frequent minerals found in concretionary forms as con- 
stituents of rocks are calcite, siderite, pyrite, marcasite and vari- 
ous forms of silica. In a true concretion the material at the 
center has been deposited first and has increased by additions 
from without, either during the formation of the enclosing rock 
or by subsequent concentration and aggregation. Where, on the 
other hand, cavities and fissures have been filled up by the depo- 
sition of materials on their walls and gradual growth inward, the 
result is known as a secretion.” 
: M. Virlet, on the 20th January, 1845, presented to the Geolog- 
ical Society of France, a paper upon this subject, in which he 
Claimed that concretionary masses had been formed subsequently 
to the deposition of the rocks or layers in which they were found, 
