2 J. A. PHILLIPS ON CONCBETIONAET PATCHES AND 



cohesion is so perfect that it is seldom difficult to obtain hand 

 specimens exhibiting the line of junction of the two. 



Such bodies sometimes enclose crystals of felspar similar in all 

 respects to those of the enclosing granite, excepting that in the 

 majority of cases their angles are more distinctly rounded; patches of 

 this kind occasionally contain others either of a lighter or of a darker 

 colour than themselves. The inclusions in granitic rocks, although 

 they have long attracted the attention of geologists, do not a^Dpear 

 to have been often a subject of investigation ; they have, however, 

 been mentioned by various authors, who have accounted for their 

 presence in different ways. 



In his ' Geological Classification of Eocks ' (p. 230), published 

 in 1821, Dr. John MaccuUoch writes as follows : — 



" The magnitude of the parts in granite is extremely various, 

 each constituent mineral sometimes exceeding an inch in di- 

 mensions, and at others being almost invisibly minute. Yarious 

 textures are also often united in a very limited space, or the rock 

 passes imperceptibly from fine- to coarse-grained. Occasionally also 

 irregular patches or veins, of a fine texture, are seen imbedded in a 

 coarser variety. In one rare instance the parts afiect a spheroidal 

 arrangement." 



!N'aumann (1858) observes : — '^ Pseudofragmentary Concretions. 

 These appear like more or less sharp-angled fragments, but without 

 being so. Such concretions occur not unfrequently in granite and 

 syenite and in other rocks made up of crystalline silicates ; they 

 have sometimes been quite erroneously interpreted, having been 

 really taken for what they seem to be " *. 



The same author subsequently remarks: — "In this connexion 

 we have yet another opinion to consider, to which attention has 

 already been called (i. p. 919), the opinion, namely, that these 

 fragments are not really to be regarded as such, but merely as 

 fragment-like concretions. That concretions really sometimes occur 

 which in their form possess a deceptive resemblance to angular or 

 rounded fragments is certainly no more to be denied than that occa- 

 sionally fragments acquire the appearance of concretions (i. pp. 422 

 and 560) by the fusion and rounding of the contours " f. 



Hochstetter states that the island of Billiton is, like Banca, 

 principally granite, and that the rocks closely resemble the stanni- 

 ferous granites of the Carlsbad district, including a porphyritic 

 granite like that of the neighbourhood of Marienbad, containing the 

 same highly micaceous fine-grained dark enclosures of globular 

 form it' 



Under the heading of '^ Segregation of Granites," Jokely makes 

 the following observations :— " Besides the lamellar, spherical, con- 

 centrically-coated, and more irregular segregations also make their 

 appearance. The spherical is usually combined with the scaled, 

 whilst the spheroidal or ellipsoidal forms which are met with in 



* C. F. Naumann, Lehrb. der Geognesie, 2. Auf. vol. i. p. 422. 



t Ibid. vol. ii. p. 203. 



X F. Hochstetter, Jahrb. k.-k. geol. Eeichsanstalt, 1858, p. 285. 



