220 ^- KOTö. 



still differ a great deal in size even in the same rock. The orthoelase 

 and plarrioclase are both of large dimensions; l)nt the latter is the smaller 

 of the two feldspars, and is idifnnorphic in comparison with the mono- 

 symmetric variety, while tlie microcline is nsually of an allotriomor- 

 phic form, and is the latest in its formation' nnd the smallest in its size. 

 Tlie orllioclase is a greyish-white form, nsnally devoid of well- 

 defined crystallographic outlines ; it becomes flesh-red in slightly 

 decomposed specimens. In the field, observers might l)e easily 

 misled to consider such aii orthoclase to be a different varietv, and 

 consequently would make a quite distinct type by calling the 

 rock which r-ontains it a red granite in contradistinction to the 

 common grey one ; but in truth both form one and the same mas.s. 

 As already stated, orthoclase is proportionally large in its size, 

 and usuîdly encloses a more or less idiom« )r])hic plagioclase ; l)ut 

 I have never met with the reverse case. l^•of. Rosenbusch 

 re]ieatedly asserts naost definitely, in his Mihroshtpische Physio- 

 (jraphie^ that thr dogma, that orthoclase falls mare ecmhj a victim 

 to décompositions than the other fellspars, is not based on ascertained 

 fact ; I)ut I geueralJy find ortlioclases to l)e more dirty, crooked, 

 and fissured, and much more decomposed than the other members of 

 the same gron.p, when viewed side by side in the same portion of a 

 slice. The mouocliuic feldspar is fissured iii all directions, and is also 

 characterized b}'- tlie presence of abundant liquid-inclusions and other 

 foreign interpositions, especially when compared with the other feld- 

 spars. AVell-defined crystals of a brown mica, indeterminable, co- 

 lourless needles looking like sillimanite, crystalloids of hornblende, 



1 Dr. Dathe, when speaking of the ordei* of crystallization of feldspars in the liiotite-g-neiss 



of the Eulengebirge, says: scheint nach meinen BeoJjachtmitji'n an.du-'<i'n und r.ahlreicheii anderen 



Gneissen und Oramiliten der Plaf/ioklas zuerxt, hierauf Mikrohlin, arldiess/icli Mikroperthit und 

 Orthokla'f sich ausuenchieden rm haben. Jahrb. d. königl. preuss. geol. Landesaustalt für 1888. p. 

 318. Whether the same rule may or may not be applied to an inidonlitedly irniptivo granite 

 like ours, is a question for the future. 



