540 EOZOON LIMESTONE OF EASTERN MASSACHUSETTS. 
devoted himself assiduously to the study of the roeks-in his neigh- 
borhood. ; 
The limestones in which the so-called Eozoon is found, as 
should be borne in mind, do not occur in a continuous line, as is 
usual in stratified rocks. They are isolated masses, often having a 
lenticular shape, and are found only at irregular intervals. Those 
at Chelmsford occupy, or rather occupied (for they have been 
largely removed) cavities, or oven-shaped spaces, formed by the 
disturbance of the overlying gneiss. In places, the gneiss now so 
overarches some of the cavities, and it evidently once so over- 
arched them all, as to make it impossible that fossils, or any other 
solid foreign bodies, could have been carried in and deposited in 
a continuous series, beneath the summits and all along the sides’ 
of the arches. 
Again, these calcareous masses have that banded structure 
which is peculiar to veins. There is, moreover, connected with 
this structure a succession of minerals, showing that the deposi- 
tion must have taken place gradually, one layer succeeding another 
from the sides of the cavities toward the centres, until the central 
Spaces were finally filled. Once more, these layers exactly con- 
form with the most abrupt irregularities of the enclosing gneiss, 
while portions of the limestone in given instances clearly ramify, 
in a vein-like way, the walls which form the cavities. 
Such are some of the points characteristic of these calcareous 
vein-stones. That the limestones of Chelmsford have truly a 
vein-like structure should be evident from what has been said, and 
especially, from an examination of the same and of similar rocks 
_as they occur in place. The question as to their origin, whether 
by infiltration, segregation, or sublimation, I leave undiscussed for 
the present. 
Now it is a fact, that genuine Eozoon — Eozoon which is recog- 
nized as genuine by those who hold to its organic nature — actu- 
ally occurs in these limestones. 
Thus, an enigma is asking for solution — an enigma involving 
difficulties not easily answered, and imposing a task not readily 
performed, in consistency with the‘commonly received views of 
this strange fossil form. Indeed, the fact that genuine Eozoon 
occurs in these calcareous vein-stones, under the conditions indi- 
cated, raises an important question which I leave those conversant 
with Eozoon structure to solve as best they are able. While it in- 
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