150 MINERALOGY AND LITHOLOGY. 



feldspar, which is usually labradorite. An oxide of iron, in the form of 

 magnetite or titanic iron, is always present, and usually, also, some green 

 chlorite which has resulted from the decomposition of the other minerals. 

 Numerous accessories are constantly or accidentally present. The rocks 

 are gray, black, or green, according to the relative amount of the ingre- 

 dients, and their condition as regards decomposition. Many of them are 

 massive, but with us the larger part are more or less porphyritic. Al- 

 though varieties abound in which large and prominent crystals are devel- 

 oped in a ground mass that is apparently very compact, still in no sense 

 is this ground mass to be confounded with an amorphous or non-crystal- 

 line body, for when magnified it is found to be entirely composed of 

 crystals, and hence these structural differences are not of the nature 

 to separate the porphyritic rocks from the others. Fundamental distinc- 

 tions are formed by the introduction of anorthite in some cases, and of 

 olivine in others. On these characters the group is sub-divided. 



Diabase (massive). Common massive diabase, as found in New 

 Hampshire, is composed of a mixture of labradorite, augite, and mag- 

 netite or titanic iron. It is nearly black when its ferruginous constitu- 

 ents predominate, light gray when the feldspar predominates, and green 

 when much chlorite is present. It varies in its texture somewhat, but is 

 usually so fine that no ingredient can with certainty be recognized by the 

 unaided eye. Its dykes occur in all parts of the state, but most abun- 

 dantly in the mountainous regions. When its thin sections are exam- 

 ined with the microscope, decomposition of one kind or another is found 

 to have made extended progress, but the modes in which this decomposi- 

 tion has progressed are very diverse, and have created quite distinct 

 types of rock. 



Chlorite is the most prominent result of one common method of de- 

 composition. In thin sections of these rocks a green chlorite is seen 

 surrounding remnants of augite. The structure and general appearance 

 of a section of this kind of diabase is represented in Fig. i on PI. 9. 

 It is made from a green-colored rock taken from a dyke in East Hano- 

 ver, and may be considered as a typical specimen of this variety. The 

 augite crystals, at the expense of which the chlorite has been formed, 

 were originally quite large, but now only little rounded grains are seen. 

 In polarized light some of them are found to be twin crystals; in ordinary 



