No. 3.] MACFARLANE— ON CRYSTALLINE ROCKS. 275 



last movement of the mass at the moment of its final solidifica- 

 tion. The observer can plainly see that this movement proceeded 

 from right to left, crowded the microlites against the right 

 sides of the larger previously formed crystals, and then carried 

 them past these in the direction of the flow, namely, towards the 

 left. The fiirure further shews that one large dark coloured 

 crystal of hornblende had been broken into two pieces, and that 

 the smallest of these, after the fracture, had been caused by the 

 motion of the mass to assume a new position against the end of 

 the larger piece. There can be no doubt, says Vogelsang, as to 

 this fact, for each piece possesses a crystalline and a fractured 

 end, and at the latter, in the larger piece, a crystal of magnetite 

 is seen which corresponds exactly to a space visible in the broken 

 end of the smaller piece. The crystal has evidently been broken 

 at this weak place, and the pieces afterwards turned and pressed 

 against each other. Sometimes the felspar crystals in this rock 

 shew a lio-ht brown ed<2;e round the clear central mass of the 

 crystal. \Yhen more strongly magnified, it becomes plain that 

 the brown vitreous matrix has penetrated the crystal in innumer- 

 able places by the cleavage planes. In some crystals this only 

 takes place to a certain depth ; others are penetrated through 

 and through by the matrix. Fluidal-structure, sometimes closely 

 resembling that just described and sometimes considerably mo- 

 dified, has been observed by Vogelsang in the basalts of Unkel 

 and Obercassel, in the lava of the island of Ischia, in the diabase 

 of Weilburg on the Sahn, in the quartzose trachyte of Cam- 

 piglia, in the black pitchstone of Zwickau, and in the quartzose 

 porphyry of Wurtzen in Saxony. Another figure gives a repre- 

 sentation of a part of the last named rock magnified 200 times. 

 In this example the Fluidal-structure is not indicated by the 

 position of crystals previously developed, but by a varied colour- 

 ing which corresponds to difi'erences of densities in the vitreous 

 matrix. A similar appearance is frequently visible in window 

 glass when its substance has not been rendered perfectly homo- 

 geneous in the manufacture. Through the whole of the matrix 

 of this rock there are scattered very fine black points, but these 

 are found much less frequently in the dark than in the light- 

 coloured portions of the matrix. 



Many of the facts observed by the naked eye, concerning 

 the order of the formation of rock minerals, are confirmed 

 by Vogelsang's researches with the microscope, Especially 



