EFFECT OF CHANGING TEMPERATURES. 283 



masses or of boulders, either large or small. Extreme changes of tem- 

 perature take place only at the surface, and disintegration due to them 

 must be most rapid there ; but, however slight the changes may be, they 

 cannot go on among the heterogeneous materials composing the rock with- 

 out causing them to slip back and forth against each other, thus pro- 

 ducing a mechanical loosening and disintegration of the! entire mass. 



The component minerals of a crystalline rock contract and expand at 

 different rates at different temperatures. Even in a single crystal the 

 contraction and expansion vary with its different axes, while the irregu- 

 lar arrangement of these crystals in the rock tends to loosen up the 

 whole mass. 



In order to comprehend the effect of varying temperature upon rocks 

 it is necessary to know of what minerals the rock is made and the coef- 

 ficients of expansion of these minerals on their different axes within the 

 range of temperatures to which the rock is subject. 



The gneisses about Rio vary a good deal locally in their composition. 

 In some places they are porphyritic, in others they are very fine grained ; 

 in some places one of the constituent minerals is in excess ; in other 

 places, others. Here they are more like granites, and there they are 

 clearly banded in structure. It follows, therefore, that changing tem- 

 peratures must affect these rocks differently according to their make-up. 



Examination of specimens of gneiss from the great quarries of the 

 Pedreira da Gloria, at the foot of the Morro de Santa Thereza, near Rua 

 da Princeza Imperial, show that it is composed of the minerals men- 

 tioned in the list below.'^ These minerals have different coefficients of 



* Description of Gneiss from the Pedreira da Gloria, Rio de Jaueii'o, Brazil. Collected by J. C. 

 Branner. 



The gneiss is an excellently banded vocl?:, and cleaves across the laminations almost like slate. 

 The banding is due to the alternation of quartzose layei's with others more feldspathic. It is 

 readily seen microscopically, and the slides bear out the inference that the dark colored silicates 

 are comparatively rare. Small red garnets are visible, but only of rounded form. 



A slide of a quartzose streak discloses quartz, orthoclase and plagioclase in this order of abun- 

 dance and a few flecks of brown biotite. The individuals are irregularly packed together; all are 

 allotriomorphic, and exhibit on their edges between crossed nicols the evidences of having been 

 subjected to dynamic strains. The feldspars show undulatory extinction, and the quartzes rims 

 of the color of their edges. Cataclastic structure or fractured edges are also to be seen finely de- 

 veloped. The quartz is abundantly supplied with inclusions, both liquid and gaseous, and an 

 occasional rutile needle is seen. The orthoclase is quite clear and transparent, and microcline is 

 also present. The plagioclase is subordinate. Biotite appears as a few small irregular flakes. 

 Little masses of apatite were noted. But the most interesting of all is the presence of sillimanite. 

 This forms small prismatic crystals, which are sometimes grouped in aggregates of two or three, 

 at other times appear singly. They ai-e about 0.2 of a millimeter broad and 0.5 of a millimeter or 

 less long. The customary parting across the c-axis is strongly developed, but the regular cleav- 

 ages are not shown. The crystals are strongly doubly refracting, and polarize in brilliant colors. 

 The crystals were isolated by means of hydrofluoric acid, and the absence of lime proved, which 

 barred out zoisite. Their strong double refraction distinguishes them from andalusite. They were 

 only seen in the slides from one specimen, where they occur in a quartzose layer. It showed no 

 tendency to form the fibrous aggregates with quartz known as fibrolite and bucholzite. Sillimanite 

 has been noted in North America by J. D. Dana in the gneiss of Manhattan island (Amer. Jour. Sci., 



