40 MINERALOGY AND LITHOLOGY. 



is always opaque. It very rarely appears crystallized, but is generally 

 in bits and patches of very irregular and indeterminate outline. It is 

 often seen in staff-like, club-shaped, and other elongated forms, and often 

 in indented and diffuse forms, which, although not sufficient to distin- 

 guish it from magnetic iron, are certainly quite characteristic of titanic 

 iron. When crystallized, it is hexagonal ; and in some of our rocks 

 hexagonal and rhomboidal plates are found which are suspected to be 

 of titanic iron. 



Although menaccanite is difficult to dissolve in acids, yet it undergoes 

 a peculiar kind of decomposition in the rocks, which is quite character- 

 istic of it. This decomposition is very often seen in microscoiDic study 

 of basic rocks. Its beginning is shown in grains that have a gray, trans- 

 lucent edge. Then, again, this gray substance traverses the black grain 

 in straight lines, following the cleavage or planes of composition ; then, 

 but a faint skeleton of black mineral is seen traversing the white decom- 

 position product ; and, finally, every trace of the titanic iron has disap- 

 peared, leaving a gray, translucent mass, which by reflected light is 

 white, and which possesses a structure dependent on the mode of its de- 

 composition. The white product resulting has been determined by Prof. 

 A. von Lasaulx to be a compound of titanic acid and lime resembling 

 perofskite [Ca Ti O3]. It is supposed that the lime of the hornblende 

 or feldspars reacts on the titanic iron, producing the titanate of lime; 

 and sometimes, when silica also takes part in the decomposition, sphene 

 may be produced. Where the iron goes to is not explained, but it 

 is likely that it enters into the composition of the ferruginous chlorites, 

 which arc so usual in these basic rocks where this mineral is most 

 common. 



The forms that the decomposition product takes, are most remark- 

 able ; and in our New Hampshire dioritcs are some more strange 

 than have been seen elsewhere. When the decomposition goes on 

 regularly from the circumference till it reaches the centre, the re- 

 sult is a mere irregular patch of translucent material, but when it fol- 

 lows the cleavage or lamination the forms are quite fantastic; and at 

 times these forms possess such a very strange similarity to organisms, 

 that they have deceived observers into the belief that they were the fos- 

 silized remnants of microscopic forms of life that existed in the original 



