350 PKOCEEDINGS 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 nug- 

 gets 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 meta- 

 morphic 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. 



YII. Supplemental Notes ots the Geology oe Nokth-west Ve- 

 nezuela. — 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 Eelipe, are contained in a mica slate, which is probably an ex- 

 tension 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 gentle- 

 man 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 Natuee and Cause of the Glacial Climate. 

 By Joseph John Murphy, Esq., F.G.S. 



In the present paper I pui'pose to show how far I agree with, and 

 where I differ from, Mr. CroU as to the views on the cause of the 

 glacial climate set forth in his paper in the * Philosophical Maga- 

 zine ' for August 1864. 



Mr. Croll's conclusions may be stated in the three following pro- 

 positions : — 



1. A glacial period occurs when the ©xcentricity of the earth's 

 orbit is at a maximum, and the solstices fall when the earth is in 

 periheUo 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. CroU 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. 



