No. 3.] MACFARLANE — ON CRYSTALLINE ROCKS. 273 



Palma, contain radiating particles of epidote which gradually 

 mero-e into the mass of the orthoclase. This and similar instances 

 can scarcely be explained otherwise than on the supposition that 

 the formation of the epidote preceded that of the orthoclase. 

 Other facts concerning the occurrence of epidote in syenitic rocks 

 would seem to indicate that the formation of the hornblende pre- 

 • ceded or took place contemporaneously with that of the epidote. 

 Senft has observed, near Brotterode, staurolite crystals enclosed 

 in transparent plates of mica, and Gr. Rose describes both stau- 

 rolite and cyanite columns as occurring in a similar manner. 

 According to Senft, tourmaline, garnet, staurolite and cyanite are 

 very constant companions of potash mica in crystalline rocks, 

 and most frequently occur bedded in it as well developed crystals, 

 and when separated from the surrounding mass of mica, leave in 

 it an accurately bounded, smooth sided and sharp angled im- 

 pression of their several forms.^' 



The order of the formation of the minerals of granite has been 

 a matter of frequent discussion, and the impression prevails that 

 the mica preceded the formation of at least the quartz in that 

 rock. Senft thus gives the result of his observations on this 

 matter : " Potash mica shews itself most frequently associated 

 " with amorphous quartz and with orthoclase ; with the first 

 " usually so that it lies imbedded in its mass, which would in- 

 " dicate a later formation for the quartz ; with the orthoclase, on 

 " the contrary, frequently so that it appears to sit upon it, so 

 " that one must regard the mica as the newest mineral. How- 

 '^ ever, there are not wanting examples of the occurrence of mica 

 " sitting upon the quartz, nor of others in which it appears so 

 " evenly intermixed with fresh orthoclase that one must ascribe 

 " to them a contemporaneous origin." f 



Senft has also the following remark on the mutual relations of 

 oligoclase and hornblende: "Where oligoclase occurs in very 

 " distinct intermixture with crystals of hornblende, it, for the 

 " most part, surrounds them, and, indeed, often completely encloses 

 " them in its mass. This relation plainly indicates that although 

 " both minerals were produced in one and the same original 

 "magma, nevertheless, the hornblende was the first born, and the 

 " oligoclase was obliged to produce itself out of that part of the 

 *' magma remaining after the formation of the hornblende." 



♦ Felsgemengthelle, p. 707, f Felsgemengtheilcj p. 707. 



Vot. VI, K No. 3. 



