LITIIOLOGY. 155 



Some sections of the labradorite porphyry from Ossipee present a 

 most interesting phenomenon. Many of the large crystals of labradorite 

 are seen to have been all broken up after they had been formed, and then 

 cemented together again. Fig. 2 on PI. 9 represents one of these crys- 

 tals. In this crystal, the bandings of color that are induced by polarized 

 light are dislocated and out of joint, while below are pieces which have 

 evidently been broken off. Other crystals in this section have been all 

 broken up into a complete mass of fragments, and then all cemented 

 together again. It appears in this case that the large crystal had grown 

 to its full size before the mass had solidified, and at some given time a 

 movement or commotion took place which broke into fragments many of 

 the crystals that had been formed, and induced at the same time some 

 change in condition, which caused quicker cooling and the solidification 

 of the residue of the matter in little crystals. A sudden change of con- 

 dition is, then, one cause which results in the production of porphyries. 



AnortJiite Diabase. The diabase of New Hampshire, in which anor- 

 thite has been proved to enter as an essential, is also porphyritic. The 

 reasons why it should be so are of a different nature from those just 

 referred to for the explanation of the structure of labradorite porphyry. 

 The essential ingredients of common diabase do not widely differ from 

 one another in fusibility ; but anorthite fuses with difficulty, and hence, 

 if it is to be formed in a mass cooling from a state of fusion, its crystals 

 will have the first opportunity to grow ; and where it is found in such 

 rocks in our state its crystals are quite large and well formed. When 

 sections are examined with polarized light, these crystals are found to be 

 more or less completely altered into an aggregate of fibres, but a well 

 defined centre is often left intact. The ground mass is usually coarser 

 than that of the labradorite porphyries. A good example of this rock 

 is found at East Hanover, in a series of small dykes that intersect the 

 slaty rocks. The anorthite is in crystals as large as hickory nuts, pos- 

 sessing quite a variety of planes (see p. 90), although these planes are 

 quite rough, as might be expected in such surroundings. The crystals 

 are commonly altered into a translucent, waxy substance, which, as al- 

 ready stated, is a mere aggregate of fine needles, and is called saussurite ; 

 but often crystals with clear and undecomposed centres are found. The 

 appearance of a thin section of one of these crystals is represented on 



