288 HENRY S. WASHINGTON 



similar rock which are met with in such abundance in the Trias- 

 sic of Connecticut and New Jersey, this being due to their having 

 cooled as much smaller bodies. Amygdaloidal structure is very 

 rare. They may be divided roughly into two main groups, the 

 ophitic and basaltic, though these merge into each other, and 

 frequently the center of a dike is ophitic while its border is 

 basaltic. 



The ophitic diabases present the usual features. The feld- 

 spar, in stout plates, is chiefly a well-twinned plagioclase, with 

 extinction angles corresponding to a labradorite of about the 

 composition Ab x An 3 . It is often cloudy or epidotized through 

 alteration. A little orthoclase seems to be present. The augite, 

 which is seldom automorphic, is pale violet-gray in thin sections, 

 and is frequently uralitized, often to such an extent that little of 

 the original mineral remains. Magnetite is quite common in 

 large grains, often showing octahedral outlines, and has a strong 

 tendency to stout skeleton growths. An interesting case of this 

 is seen in a dike-cutting rhyolite on Marblehead Neck where 

 the magnetite skeletons assume the form of small stout crosses 

 with thickened ends, or with their ends joined by the sides of a 

 hollow square, the cross in this case forming the diagonals. 

 These growths are analogous to those of leucite in certain leuci- 

 tites from Montana 1 and Italy. 2 The magnetites are frequently 

 accompanied or surrounded by brown, apparently secondary, 

 biotite, even in the freshest specimens. With this exception 

 neither biotite nor hornblende is to be seen, nor was olivine 

 observed. Apatite is not abundant. 



The basaltic diabases are black and aphanitic, without mega- 

 scopic phenocrysts. They show in thin sections laths of clear 

 labradorite and some crystals of augite in a mixture of augite 

 grains, small labradorite laths and magnetite with considerable 

 light-brown glass base. . The magnetite very frequently assumes 

 delicate arborescent forms, branching at right angles, which are 

 very pretty and characteristic. In a small apophysis of the 



T L. V. Pirsson, Bearpaw Mountains, Am. Jour. Sci. (4), Vol. II, p. 145, 1896. 

 2 H. S. Washington, Bolsena, Jour. Geol., Vol. IV, p. 557, 1896. 



