168 The American Geologist. September, 1&94 
masses, that this point has almost wholly lost the significance 
it once possessed. By some, however, the mineral chondro- 
dite is still used in this way with the force and rank of a 
characteristic fossil, since it occurs abundantly in certain 
phases of the white limestone. 
Prof. J. D. Dana* has well observed in a paper on certain 
rocks of the Connecticut valley: "What reason is there in 
chemistry or geology why crystals of andalusite and stauro- 
lite should have been made only in pre-Silurian time? Anda- 
lusite consists simply of alumina and silica, and staurolite of 
the same along with iron. These three ingredients are now 
and ever have been the most abundant of all the mineral con- 
stituents of the globe." 
To these chondrodite might well be added, along with all 
other minerals, since their production depends upon a union 
of certain chemical elements under the proper physical con- 
ditions and is in no wise a function of geological age. 
In regard to chondrodite, composed as it is of silica, alum- 
ina, magnesia, iron and fluorine, no one would claim that its 
elements were Archaean or restricted to Archaean formations, 
unless, possibly, it might be the fluorine. This latter element, 
however, along with boric acid, is well known to be one of 
the most constant accompaniments of igneous magmas, re- 
vealing itself in the composition of various minerals formed 
by fumarole action. Many instances of this might be given, 
but, as geological literature is full of such, it will be sufficient 
to cite the well known occurrence of topaz in the cavities of 
rhyolites in Colorado and Mexico; while the actual presence 
of hydrofluoric acid in gaseous emanations from the fuma- 
roles in the crater of Volcano has been shown by actual 
chemical tests by Seacchi.f and various volatile fluorides are 
known to occur after eruptions as efflorescences at Vesuvius. 
Moreover, the fact that chondrodite is not always of Ar- 
chaean age is proved by its well known occurrence at Mt. 
Vesuvius in cavities of the ejected blocks of the Cretaceous 
dolomite which forms the pediment of the volcano. Its forma- 
tion there is evidently due to the action of mineralizing vapor, 
*Am. Jour. Sci.. III. vol. vi. p. 350, 1873. 
+ A 1 1 . Ace. Napoli, •'>. 1873, ('out. Min. II, is?4. 
