790 HENRV S. WASHINGTON 
very fully by Shaler. Judging from the evidence of strain 
revealed by the microscope the rifting (and possibly the joint- 
ing in part) is largely due to dynamic action subsequent to the 
consolidation of the mass. 
In thin sections are seen the following minerals; orthoclase, 
albite, microcline, hornblende, glaucophane, biotite, pyroxene, 
magnetite, zircon, apatite, and allanite. 
The quartz is highly xenomorphic, being usually interstitial. 
In some cases (Bass Rock, Rocky Neck at Gloucester) it also 
occurs in small rounded anhedra, scattered through feldspar 
areas. A brecciated structure at the borders of the larger 
quartzes is not rare, and is associated with an undulatory extinc- 
tion. Mineral inclusions are few, and are only zircons.. Gas 
and liquid inclusions, the latter carrying a movable bubble, are 
quite abundant though small, and occur in streaks. 
The alkali-feldspars are varied and deserve more extended 
study than the scope of this paper allows. They are chiefly 
orthoclase and albite, which are almost always intergrown so as 
to torm highly typical microperthites and cryptoperthites. In 
general they are closely similar to those described by Brégger? 
and Ussing? from Norway and Greenland syenitic rocks, and 
their figures would answer for forms seen here. Cryptoperthitic 
development is relatively scarce, the majority being perthitic 
on a fairly large scale. Mlicrocline also is not rare, but less 
abundant than the preceding. This and the orthoclase are 
apparently rich in soda. The feldspars are apt to be rather 
cloudy and dusty with decomposition products, and they often 
show a brecciated border and undulatory extinction. Inclusions 
of apatite and zircon are rare. A very little albite-oligoclase 
is seen in a few sections, showing well-developed twinning 
lamelle. 
Hornblende, the chief ferromagnesian mineral, is usually 
xenophorphic, crystal planes being seldom seen. It is in gen- 
‘ SHALER, 9th Ann. Rep. U. S. G. Surv., pp. 583-588, 602-605, 1889. 
2W.C. BrOcGER, Zeit. Kryst., Vol. XVI, pp. 521-560, 1890, Pls. XXII, XXIII 
3 UssinG, Medd. om Groénland, Vol. XIV, pp. 5-101, 1898, Pls. I, II, III. 
