ORIGIN OF MINERALS CHARACTERISTIC OF OPHITES ETC. 69 



The cleavage of hornblende and augite is prismatic and basal : 

 the two prismatic sets are parallel with the faces of the secondary 

 rhombic prism ; while the basal set, which is parallel with the 

 basal faces of the inclined primaries, necessarily intersects the 

 prismatic cleavage-planes obliquely *. 



Obviously, whatever difficulty attaches to crystalline form, 

 cleavage is of great assistance in enabling the investigator to 

 distinguish peridote from either augite or hornblende ; for while 

 there are only right-angled sets of cleavage in the first, there 

 are nothing but oblique sets in the two last. Still there are 

 difficulties to be encountered in connexion with this point, 

 depending on whether or not the section which may be under 

 examination is in its correct, or approximately correct, position 

 of intersection relatively to the vertical axis of the prism : but 

 their consideration may be advantageously passed over as in- 

 volving niceties of calculation unnecessary in this work; and 

 we feel ourselves bound to argue out our conclusions suitably 

 for geological students in general. 



Let us next examine the sections of crystals of " peridote/' 

 as represented by two of our fellow labourers, to see how far they 

 are in conformity with the cleavage-characters of this mineral. 



Unfortunately the figures of " peridote " given by Allport " f, 

 although representing each a cross section, do not exhibit any 

 cleavage. Professor HulPs figures are not in this predica- 

 ment J. 



Allport, who has carefully examined a number of British 

 dolerites, has represented in his figures 25 and 27 cross sections 

 of what he takes to be peridote ; but, from their form, it may be 

 equally assumed that they represent augite. He has also given 

 another section, four-sided, in fig. 26; but exception may be 

 taken to it as being too simple in form for peridote, and 

 having its angles approaching too closely to those characteristic 

 of the acute rhombic modification peculiar to hornblende in its 

 simplest secondary form, and as equally represented by its clea- 



* The basal cleavage of these minerals, it will be understood, could not 

 be represented in figures of cross sections. 



t Quart. Journ. Geol. Soc. vol. xxx. pi. 34. figs. 25-27. 



\ Trans. Royal Irish Academy, vol. xxv. pi. xi. figs. 25-30, u Report on 

 the Chemical, Mineralogical, and Microscopic Characters of the Lavas of 

 Vesuvius from 1631 to 1868," by the Rev, Prof. Haughton and Prof. Hull, 



