LITHOLOGY. 203 



occurrence in the Notch, is composed of orthoclase, a httle quartz, and 

 many garnets, and is in composition like granuHte, which, however, is 

 a fine-grained and stratified rock. 



A variety from Rye is very feldspathic, but when a thin section is 

 examined quartz is found, forming a cement that fills all the inter- 

 spaces between the well formed and often twinned feldspar crystals. As 

 an illustration of the primary formation of the orthoclase, a section of 

 this rock is interesting. In Concord, just over the border, in Vermont, 

 another example of exactly the same kind is found. These rocks, being 

 essentially all orthoclase, furnish an example of a mineral as a rock. 

 Masses of granular orthoclase in connection with granitic rocks have 

 been often before observed. 



Pudding Granite. A most peculiar granite from Craftsbury, in Ver- 

 mont, is included in the collections of the survey, and has been examined. 

 This is a biotite muscovite granite, which contains concretions of biotite 

 that are quite uniform in size, and usually about an inch and a half in 

 diameter. They are spherical or spheroidal in form, and corrugated on 

 the surface ; and these black shining balls, scattered through the massive 

 and light-colored rock, impart to it a most striking appearance. This 

 granite is well known and widely celebrated as the pudding granite, a 

 name indeed very appropriately suggestive. Desiring to know what 

 could form the nucleus of these spheres, I sliced one of them through the 

 middle, and of one half made a thin section which contained the centre. 

 The interior of this concretion, as a thin section of it appears when 

 slightly magnified, is represented in Fig. 4 on PL xi. The whole mass of 

 the concretion is composed of strongly dichroic biotite, a little muscovite, 

 and quartz. The section shows that it has nothing that can be called a 

 nucleus, but is only a concretion of mica scales which began to be laid 

 concentrically as soon as the first irregular beginning had grown suffi- 

 ciently to form a basis. Zepharowich* has described such concretions, 

 that occur in the mica schist of Herrmannsschlag in Austria, which in 

 the interior are composed of concentric layers, as in this case ; but when 

 the concretions were half grown a sudden change took place, and the 

 mica began to arrange itself radially, and so continued till the growth 

 ceased. 



* Mineralogical Lexicon, Austria, p. 59. 



