SCIENTIFIC SUMMARY. 
211 
the temperature of the two hemispheres.” Mr. Croll gives a long series of 
tables showing the different values of the excentricities, at different epochs 
and from them calculates the dates of the Glacial periods. 
Nests of altei'ed rock in the grey Granites of the Southern Uplands. — The 
subject of the metamorphosis of rocks is now occupying geologists, and the? 
memoir lately contributed to the Geological Magazine by Mr. James Geikie 
must therefore be of great interest to our readers. After dwelling at some 
length on the metamorphosis of aqueous rocks as shown in certain specimens 
which he examined, Mr. Geikie passed on to the above subject. These nests 
he says, must be familiar to all who have ever travelled over large granite 
districts. They consist of a fine-grained or semicrystalline dark baked-like 
rock, which is enclosed in a granitic basis. Frequently they are seen to 
show traces of lamination, but their most usual character is that of exceed- 
ingly fine-grained mica-schist whose dark or blackish hue is due to the 
large quantity of mica present. Most commonly, says Mr. Geikie, they are 
“ of very irregular shapes, and are by no means confined to those portions of 
the rock that abut upon the outlying bedded or aqueous strata, but are scat- 
tered indiscriminately throughout the granite. The granite and the rock 
of a nest are firmly knit together, so that when a suitable edge has been ob- 
tained, a smart tap with the hammer will fetch away a good specimen to 
show the junction. This is commonly so marked that one may place a 
knife-edge upon it ; but I have sometimes (though rarely) met with u nests ” 
the fine-grained or compact rock of which seemed to pass by insensible gra- 
dations, both of colour and texture, into the outside granite. As far as my 
observation goes, the “ nests ” are, as a rule, harder, tougher, and less easily 
weathered than the granitic matrix, so that on exposed surfaces the “nests ” 
usually stand out in relief. In some granites and granitoid rocks, however, 
the reverse is the case, the “ nests ” decomposing out and leaving behind 
them little pits and irregular hollows. When the granite and the contained 
nests are of the same or nearly similar hardness, it sometimes happens that 
decomposition has set in along the line that separates the one from the other, 
affecting both equally.” Vide Geological Magazine , No. 30. 
Origin of Petroleum. — Although nearly all geologists are agreed as to the 
organic origin of Petroleum, a great many are of opinion that the rock-oil is 
the result of a natural distillation of coal. Professor Hitchcock, however, 
no mean authority, comes to a different conclusion. Admitting, with all who 
have carefully studied the matter, that Petroleum is of organic origin, he 
says, that in his opinion it comes from plants, and that it is not, as some have 
suggested, a fish-oil or a substance altered to adipocere. It does not appear 
to be the result of a natural distillation of coal, since its chemical composi- 
tion is different from the oil manufactured artificially from the cannels, con- 
taining neither nitro-benzole nor aniline. Moreover, petroleum occupied 
fissures in the Silurian and Devonian strata long before the trees of the Coal 
period were growing in their native forests. The nearly universal associa- 
tion of brine with petroleum, and the fact of the slight solubility of hydro- 
carbons in fresh, but insolubility in salt water, excite the inquiry whether 
the salt-water of primaeval lagoons may not have prevented the escape of 
the vegetable gases beneath, and condensed them into liquids. 
The Rev. W. S. Symonds and the Belgian Bone Caves. — We are sorry to be 
