206 MINERALOGY AND LITHOLOGY. 



biotite, magnetite, and pyrite. Like the specimen last described, the 

 augite is in process of transformation into hornblende; but in this case 

 the resultant hornblende is not the green, fibrous, impure variety that is 

 common under such circumstances, but it is a compact, homogeneous, 

 brown, dichroic kind. The external hornblende is as pure and possesses 

 as definite a cleavage as the augite within ; and when the plane of the 

 section intersects a grain at right angles to the vertical axis, the cleav- 

 ages of the two minerals bear the same relation to one another as do 

 the faces of their prisms, if constructed upon the same clino-diagonal as 

 has been explained on page 57. Fig. i on PI. 7 is drawn from a section 

 of this rock, and shows a grain cut parallel to the base, and another cut 

 parallel to the prism. The dichroism is much more marked when one 

 Nicol is upon the instrument. 



An augite sienite occurs upon Little Ascutney mountain. In a speci- 

 men from Columbia, biotite becomes a prominent ingredient. In the 

 last case, some grains of augite are nearly intact; but it may be said, 

 that in New Hampshire the augite sienites form a little group of rocks 

 which only microscopic study could have identified, since the original 

 characters are shown by the study of the cores of the mineral grains. 



Hornblende Sienite. The members of this group are very diverse in 

 their appearance, but are easily recognized, and present some very inter- 

 esting characters. The augite sienites are entirely free from quartz; 

 but in the hornblende sienites the microscope usually detects a little, 

 and its presence indicates the relationship of these rocks to the granites. 

 A beautiful variety comes from Red hill in Moultonborough. It is com- 

 posed essentially of orthoclase, which exists in thin, tabular twinned 

 crystal, which mostly He in one plane, and consequently give to little 

 specimens of the rock a stratified appearance. The hornblende, which 

 is irregularly distributed, is black, but in thin sections it is deep yellow. 

 and it encloses more or less biotite in its mass. Microscopic grains of 

 blood-red hematite and black magnetite, and crystals of apatite, are de- 

 tected, and by aid of polarized light some plagioclase is found to be 

 present. Only a very little quartz is seen in some little angular corners 

 made by the meeting of the straight edges of the orthoclase crystals. 

 Little, partially crystallized grains of sphene arc found; and some of the 

 grains of hornblende are shown by polarized light to consist of two parts 



