The Paragenetic Relations of Minerals. 339 



The above mentioned transfer of mispikel, from a lode 

 into gneiss, is an analogous fact, the more recent lode mass 

 having acted upon a pre-existing rock, in the same way as a 

 more recent rock mass. A number of other instances might 

 be brought forward, but as there are no facts at variance 

 with this proposition, it is unnecessary. 



6. When the crystalline variety of a mineral is associated 

 with the compact variety, the former is always the most re- 

 cent. 



Many porphyries consist of a mixture of compact felsite, 

 with imbedded crystals of cleavable felsite. There is, there- 

 fore, no reason to deny that the latter are of more recent 

 formation than their matrix. In granular or slaty mixed 

 rocks large crystals, of some one or more constituents, are 

 sometimes imbedded, communicating to the rock a porphy- 

 ritic character ; and there can be little doubt that such 

 crystals are of more recent date, for not only do they contain 

 nuclei of the rock in which they lie, but in the slaty rocks, 

 the curvature of the layers surrounding the crystals is likewise 

 to be observed, as in the gneiss of Schwartzenberg (Saxony). 

 Many minerals occur imbedded in the form, not of crystals 

 but of nodules. In. other respects they correspond precisely 

 with the inclosures in porphyritic rocks, and there is no doubt 

 that they have often originated in a similar manner. This is 

 very evident in the case of iron pyrites, which frequently 

 occurs, both in nodules and crystals, in the same bed of clay. 

 Nodular iron pyrites occurs in all rocks up to the clay-slate. 

 It is probable that many other nodular minerals must be re- 

 garded as of later formation than the masses in which they 

 lie, although water- worn fragments are also imbedded in this 

 shape. Again, agate occurs in the form of nodules, although 

 it has been formed in a very different manner. Consequently 

 very great caution must be exercised in forming any inferences 

 with regard to the origin of imbedded nodules. Nodular mi- 

 nerals are sometimes associated with imbedded crystals, and 

 this is then additional evidence in favour of their analogous 

 origin, thus sphaerulite occurs in pitchstone, pearlstone, and 

 obsidian, together with crystals of astrite and felsite. The 

 nodules of azurite, in the marl and sandstone of Miedezana 



