MINERALOGV. 43 



cord, Vt. All these forms are, however, quite common, and are often 

 observed in the trap rocks. The delicate arborescent forms like the last 

 appear, with high magnifying power, to be composed of crystals; but the 

 form of the crystals is disguised by the presence of myriads of minute 

 translucent crystals that have attached themselves to them. 



These skeletons of magnetite, so regularly formed, indicate that the 

 magnetite was the first mineral to form in these rocks, since such delicate 

 yet determinate forms could only develop in a quite plastic mass. Some- 

 times hexagonal sections are found, which may be sections of dodecahe- 

 drons ; but in such cases it is not easy to decide whether it is magnetite 

 or titanic iron. It may be stated, however, that magnetite is more often 

 crystallized than titanic iron, and is usually easy to recognize by its form ; 

 and when without definite form it is more often in compact grains, and 

 does not show the peculiar decomposition to which titanic iron is subject. 



At times, particles of magnetite are grouped in regular forms not de- 

 pendent on its own crystallization, but on that of some other mineral. 

 For example : in sections of some of our diorites we see that the horn- 

 blende has been well-nigh entirely decomposed, and though possessing 

 still its original form, it is now composed of an aggregate of three or four 

 other minerals, among which magnetite is at times predominant. When 

 magnetite has thus resulted from the decomposition of a ferruginous min- 

 eral, like hornblende, the individual particles are often grouped together in 

 forms resulting from the outline or the cleavage of the hornblende. Fig. 

 3 on PI. 2 is drawn from a section of the eruptive diorite from near the 

 Profile house. This section is cut parallel to the base of the original 

 hornblende crystal; and the magnetite is arranged along lines parallel 

 to its cleavage. In other crystals the magnetite surrounds the edge, in 

 a regular line, and is irregularly scattered through the interior. Some- 

 times, again, a row of magnetite grains surrounds the outside boundary 

 of an apparently intact crystal. The figure given is, however, sufficient 

 to illustrate this subject. A high magnifying power does not show that 

 these particles are crystalline. 



Magnetic iron decomposes with difficulty; but its grains in rocks are 

 often seen surrounded by a yellow ring of the hydrous sesquioxide of 

 iron. 



The beds of magnetite, such as exist in our state, are supposed to result 



