THE AMERICAN MUSEUM JOURNAL 



ORBICULAR GRANITE FROM MICHIGAN. 



TWO-THOUSAND pound mass of the rare kind of 

 granite known as Orl)icukir, or Spheroidal, Granite 

 was placed on exhibition in the Hall of Geology in 

 I February. This kind of granite, which is an ig- 

 neous rock, results from the peculiar conditions 

 that obtained while the original mass was cooling from a state of 

 fusion. Substances of similar chemical composition tend to 

 separate out from such a cooling mass and to form more or less 

 rounded, concretionary balls, which lie in a matrix formed of the 

 remainder of the magma. The resulting rock looks like a con- 

 glomerate, or " pudding-stone," but is entirely different in charac- 

 ter. Conglomerate is a sedimentary rock, and the rounded masses 

 in it are pebbles which have been ground into shape by the action 

 of water, and which are now in a matrix of similar origin. Or- 

 bicular granites and diorites, another kind of igneous rock, have 

 been found in New Hampshire, Rhode Island, Scandinavia, Fin- 

 land, Corsica, Ireland and elsewhere. The block now under con- 

 sideration is a boulder which was found near Charlevoix, Michigan. 

 Its original source is not known, but the boulder must have been 

 brought by the ice of the Glacial Epoch from some ledge to the 

 north, probably in Canada. 



Those who have been following the announcements of dis- 

 coveries regarding radium and the radio-activity of elements and 

 minerals will examine with interest the special exhibit of about 

 seventy species and varieties of minerals which has been assem- 

 bled by the Department of Mineralogy, for the purpose of showing 

 the substances containing more or less of the element uranium. 

 All these minerals have been stated to show radio-activity or to 

 be responsive to radio-active substances. 



Noteworthy additions to the mineral collection are three 

 groups of twinned crystals of calcite from Joplin, Mo. The ma- 

 terial is amethystine in color, and the crystals are remarkable for 

 size and clearness. One crystal is fourteen inches across. 



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