350 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9,
miners. In the mining district it is in the form of vesicular masses,
of one or two cubic feet contents as a maximum. It frequently
cements fragments of vein-quartz to form a breccia; and small nuggets
of gold are not unfrequently found imbedded in it: even when
occurring dissociated from these masses, they are more or less
coated with the mineral. On the gneissic surface the ferruginous
agglomerations do not exceed the size of an orange, and usually
enclose rounded pebbles. The origin of this mineral is to be sought
for in the decomposition of the iron-pyrites of the lodes and that of
the ferruginous minerals that enter into the composition of the metamorphic
rocks. The limonite abounds on the north slope of the ridge,
carrying the Tigre lode, and in parts of the Potosi estate, in which
instances they are in proximity to pyritous lodes. On the gneissic
area they are more abundant between Guacaima and Upata, where
the gneiss is hornblendic. The greater abundance of this mineral
in the mining-district is to be ascribed to the relatively larger store
of iron in the rocks constituting this tract.
VII. Supplemental Notes on the Geology of North-west Venezuela. —My friend, F. Mathews, Esq., C.E., has presented to the Society a series of minerals and rock-specimens from the province of Cora, from which we learn that the cupriferous veins of Aroa, near San Felipe, are contained in a mica slate, which is probably an extension of the metamorphic series of Valencia described by Mr. Wall. Specimens of lignite and brown coal point to the existence of the Upper Miocene in the province of Cora. And this gentleman has noted the occurrence of a shell-bed at Tucacas, extending three miles inland, and attaining an elevation of 30 feet. The shells are species of Tellina, Cardium, Venus, &c., and are identical with those of the present beach.
5. On the Nature and Cause of the Glacial Climate. By Joseph John Murphy, Esq., F.G.S.
In the present paper I purpose to show how far I agree with, and where I differ from, Mr. Croll as to the views on the cause of the glacial climate set forth in his paper in the 'Philosophical Magazine' for August 1864.
Mr. Croll's conclusions may be stated in the three following propositions:—
1. A glacial period occurs when the excentricity of the earth's orbit is at a maximum, and the solstices fall when the earth is in perihelio and in aphelio.
2. Only one hemisphere, the northern or the southern, has a glacial climate at the same time.
3. The glaciated hemisphere is that of which the winter occurs in aphelio.
I agree with Mr. Croll as to the first two propositions, but differ as to the third. I believe that the glaciated hemisphere is that of which the summer occurs in aphelio.
