Mr. Skene on the Emigration of Caterpillars. 9£5 



bohedral Corundum, or in fact to all those which seem to 

 possess a more distinct cleavage than other varieties of the 

 same species. The fracture of the films engaged in the mass 

 is generally conchoidal or uneven, and only occasionally a 

 small part of an even face will betray the reverse situation of 

 these plates, which, if any, must take place if the cleavage itself 

 be at least very indistinct in that direction ; and the composi- 

 tion cannot therefore assume that remarkable appearance of 

 the very obtuse re-entering angles, observable in cleavage, 

 which distinguishes albite from felspar. Upon the face of 

 R — cd, this composition produces striae crossing each other 

 at angles of 60° and 120°. Sometimes we observe these striae 

 only in one or in two directions ; and in the same manner also 

 sometimes only one, sometimes two of the faces of R are ob- 

 tained with greater facility, by breaking a mass of Corundum, 

 than the rest, which present a glassy conchoidal fracture. It 

 is worth noticing, that the isomorphism of the two oxides of 

 alumina and of iron extends even to this occurrence of regular 

 composition, which at first sight would appear to be entirely 

 accidental. The last mode of composition also occurs in 

 Chabasie, two rhombohedrons being joined in one of their 

 faces. It has been observed in the varieties from Fassa, and 

 in those from Faroe, which accompany Mesole and Apophyl- 

 lite. 



(To be continued.) 



Art. XV, — On the Emigration of a Colony of 'Caterpillar s, * 

 observed in Provence. From the MS. Tour of James 

 Skene, Esq. of Rubieslaw. 



In scrambling over one of the arid coteaux above Tolonai, 

 the beautiful summer residence of our worthy old friend, 

 Marshal Comte Gallifet, I was attracted by the manoeuvres 

 of a troop of emigrating insects, which amused me very much. 

 It is very easy to attribute the singular economy in the ac- 

 tions of the insect world to the mere influence of instinct, as 

 the governing principle of every living thing below the scale 

 of reason ; but we must either extend the meaning of that 

 word beyond the mere actions of an involuntary impulse, or 



* This is probably the Phalama processionea of Linnaeus. 



