1839] 



Trap Dykes in the Sienite of Amhoor. 



293 



action of a furnace till fusion took place, when it was removed, and al- 

 lowed to cool, he thus obtained, synlhetically, those very minerals, in a 

 state quite analogous to that in wliich they had been found in the na- 

 tural world. Having in this manner combined the elements of mica,* 

 he succeeded in forming beautiful, and distinct crystals from them, in 

 this case, finding it essential, that the fused mass should cool very 

 slowly. This remark is very interesting as connected with the formation 

 of sienite, granite, &c. in which mica occurs so abundantly. Similarly, 

 having combined the elements ofaugitet, he succeeded in obtaining it in 

 a crystalline form, more rapid cooling being requisite for it than for 

 mica. This also is quite consistent with the observations of 

 mineralogists, who remark that augite is always found associated 

 with such rocks, as have experienced a rapid decrease of temperature. 

 So material is the influence exerted by rapid cooling, on the nature of 

 certain minerals, and especially on augite, that it is often the only cause 

 of their differino; from certain others. Thus in reference to auo;ite, its 

 composition, and many of its external characters, are precisely similar to 

 those of hornblende, so much so, that Professor Rose of Berlin has pro- 

 posed, to consider them as one and the same mineral . 



Since hornblende enters so largely into the composition of all trap rocks, 

 and especially abounds in the trap of Amboor, the following remarks, re- 

 lative to the cause of the difference between it and augite, are very in- 

 teresting : — " It is well known," says Allen, from whose useful work on 

 mineralogy I quote "that augite and hornblende seldom occur together: 

 and that when they do, it is in trachylic rocks and lavas of later 

 date, in which the one forms crystals in the cavities, the latter, 

 a portion of the mass. The frequency of augitic forms, and the 

 non-occurrence of hornblende, among crystalline slags — the results 

 of Mitscherlich and Bertheor's experiments in producing augite ar- 

 tificially, whereas they never succeeded in forming hornblende, and 

 lastly, the general occurrence of hornblende, associated with quartz^ 

 felspar, &c., and such rocks as have experienced a slow decrease 

 of temperature, while augite is always associated with others as oli- 

 vine which are known to have cooled rai)idly— all concurred, in lead- 

 ing Rose to conclude, that the crystalline form presented by each of these 

 substances, depended entirely on the more or less rapid cooling to which 

 they had been exposed. Upon fusing in consequence, a mass of horn- 

 blende in a porcelain furnace, he found that it did not in cooling, assume 

 its previous shape, but invariably took that of augite." Such being the 



* Bucliland's Bridgewater Treatise, 

 i do, do. do. 



