456 Scientific Geology. 



The sienite of this valley sometimes exhibits a strong disposition 

 to assume a columnar form. Perhaps this is exhibited no where to 

 better advantage than in the ledge by the road side about a mile north 

 of Northampton village. The columnar masses are only a few inches 

 in diameter, and are much less perfect than those found in green- 

 stone. The fact, however, is interesting, as indicating a similarity 

 in the causes that produced the two rocks. 



Pseudo- Stratification of Sienite. 



Atone of the quarries of this rock at Sandy Bay, Cape Ann, on 

 the road from thence to Squam, a remarkably fine example may be 

 seen of the division of this rock into parallel portions. Their thick- 

 ness varies from two inches to two feet, and great facility is thus af- 

 forded for quarrying the stone. 



As this apparent stratification extends only a few rods, while all the 

 rest of the sienite on the Cape that I have met with, is unstratified, we 

 cannot regard this case as real stratification. I consider it an example of 

 the concretionary structure on a large scale. But it is unnecessary in 

 this place to dwell upon this explanation, since I have already discussed 

 it in treating of a similar phenomenon in greenstone. 



Veins in Sienite. 



These are numerous and various in their probable mode of produc- 

 tion, as well as in their composition. Some of them might properly 

 be denominated, " veins of segregation ;"* "as they seem to have been 

 formed by a separation of parts during the gradual passage of the min- 

 eral masses into a solid state." In most cases they appear to consist 

 only of harder portions of the rock, which become visible only by 

 the weathering of the surface, when they are left in projecting ridges 

 and at a little distance cannot be distinguished from injected veins. 

 I presume that it will be found in all cases, that these veins differ 

 somewhat in composition from the rest of the rock : and indeed, in 

 some cases this is obvious, as they contain more or less of a foreign 

 mineral, such as epidote or quartz. 



The greater part of the veins in our sienite consist of materials 

 foreign to the nature of the rock, and introduced subsequently to its 

 original production. I do not say subsequently to its consolidation : 

 for it has appeared to me possible, that while a molten mass of rock, 



* Sedgwick's Anniversary Address before the London Geol. Soc. 1831, p. 3. 



