486 
coming months, the problem of forecasting the 
character of a season would be capable of solution. 
Wate 
OLIVINE ROCKS OF NORTH CAROLINA. 
Mucu interest was attracted a number of years 
ago to the olivine rocks of North Carolina by the 
excellent paper of Dr. Genth, on ‘ Corundum and its 
alterations.’ These rocks are also well known prac- 
tically from their association with the mica and 
corundum mines of that state: hence any thing tend- 
ing to elucidate the origin and history of these im- 
mense masses of olivine is of value. There has been 
recently published, in the Proceedings of the Boston 
society of natural history, a paper by Dr. Alexis A. 
Julien on these olivine rocks, which is of great value, 
even if some exceptions may be taken to his conclu- 
sions. The particular variety of olivine rock in 
North Carolina is designated as dunite; it having 
been named from Mount Dun in New Zealand, from 
which locality rock of this character was first de- 
scribed. In North Carolina the rock is found in 
oval or lenticular masses in a hornblendic gneiss; and 
a ‘marked slaty lamination’ is looked upon by Dr. 
Julien as stratification which dips at a steep angle. 
His reasons for regarding this banded structure as 
bedding-planes are, that, on microscopic study of thin 
slices, there is seen an alternation of coarser and finer 
irregular grains. Again: grains of chromic iron are 
found not only dispersed throughout the rock-mass, 
but also in thin bands alternating with the olivine 
bands. He found, however, a sharp break between 
the lamination of the olivine rock and the foliation 
of the hornblendic gneiss surrounding it. Again: 
when there has not been formed in the rock some 
material, of later date than the time the rock came 
into place, which serves as a cement to hold the oli- 
vine grains together, the rock is pulverulent and fria- 
ble, like a loosely consolidated sand. 
From the above, Dr. Julien draws the conclusion 
that this dunite is neither of chemical nor of erup- 
tive origin, but rather an accumulation of débris from 
some older olivine rock of eruptive origin; that is, 
it is an olivine sandstone. The chief defect in Dr. 
Julien’s reasoning is, that all the evidence which 
he gives in support of this view could exist equally 
well if the rock had some entirely different origin. 
In order to prove that any thing must have been 
formed in any particular way, we ought to seek 
for certain characters in it which could have been 
produced in that way alone. 
Messrs. W. C. Kerr and C. D. Smith, who have 
spent much time in studying this olivine rock in the 
field, declare in favor of the eruptive origin of it; 
but they have published little or none of the evidence 
upon which their conclusion rests, and therefore one 
cannot judge as to its correctness. Every rock car- 
ries within itself, or in its relations to others, the 
story of its origin and subsequent history with more 
or less completeness. The correct reading of that 
story depends upon our skill and knowledge. If a 
rock is deposited in the hollows of another as a beach 
SCIENCE, 
ee ie Owe eee 
[Vou. III., No. 63. 
formation, it is easy to see that the effect it produces 
upon the boundary-rock is different from its action 
upon them as a lava-flow or an intrusive mass. So 
the last two cases present different relations, accord- 
ing to their origin, to the surrounding rocks. As 
a rule, it can hardly be considered safe to posi- 
tively declare what the origin of an old crystal- 
line rock is, until these relations have been carefully 
ascertained; and in this direction Dr. Julien’s work 
is defective. The present writer’s microscopic study 
of the North Carolina dunite showed him that the 
rock he was studying, even when destitute of some 
cementing-material, was not friable and pulverulent, 
while the sections to his mind presented characters 
belonging to eruptive rocks only. ‘The olivine grains 
are separated by fine fissures, but every irregularity 
in the outline of one is matched by a corresponding 
irregularity in the adjacent bounding-grains. If 
these grains had been water or wind worn olivine 
sand, no such matching of the parts would have been 
possible. This any one can readily see for himself 
if he will examine any conglomerate, and observe 
the amount of interstitial material it takes to hold 
together and fit the pebbles to one another. Then 
let him remember that a sandstone is a conglomerate 
on a small scale, and, under the microscope, a con- 
glomerate to the eye as much as the other is to the 
unaided vision. The olivine rock now under con- 
sideration has absolutely no interstitial spaces and 
no binding-material, but the grains are fissured and 
separated the same as the adjacent portions are sepa- 
rated in cracked and fissured glass. From this the 
conclusion naturally follows, that such structure 
indicates that these olivine grains were formed by 
the cracking of an olivine mass during the process 
of solidification, crystallization, and cooling; that 
is, from an eruptive mass. 
Further, individuals of olivine are seen in polarized 
light to be made up of a number of distinct grains, 
as much separated by fissures from one another as 
the distinct individual grains are elsewhere in the 
section. ‘This is a natural and common occurrence 
in an eruptive rock, but in a sedimentary one the 
parts ought to be scattered. Many of these indi- 
viduals, too, are long, wedge-shaped masses with 
sharply pointed ends. If they had been water or 
wind worn grains they ought to have had these 
sharp edges worn, rounded, and broken. ‘These long, 
lenticular, fissured individuals are also arranged at 
every angle to one another, when, if the rock were 
sedimentary, they ought to lie nearly parallel, and 
on their sides. 
The alterations of the dunite described by Dr. 
Julien are important and interesting because they 
give rise to veins and other rocks. The corundum 
in these veins is looked upon as a secondary product, | 
and not, as Dr. Genth held, the primary material 
from which many rocks originated. The change of | 
the olivine rock to different rocks leads to the pro- 
duction of chalcedonic or cherty forms, hornblendic 
schists, tale schists, serpentine, etc. ‘The change to 
serpentine comprises every variation, ‘‘ from that in 
which the serpentine is diffused among the olivine 
