THE IGNEOUS ROCKS. 523 



and boss masses are very similar in the nature of tlieir material. Both com- 

 prise schistose forms of diabase, in which all, or nearly all, of the au<nte 

 has been changed to green hornblende. Along the peripheries of the boss 

 masses and throughout many of the dikes the rocks are very schistose. 

 They have likewise suffered great changes in composition, and are now 

 often chlorite-schists or talc-schists 



In the eastern portion of the area nearly all the boss greenstones have 

 the characteristics just mentioned, but in the western bosses the rocks 

 have suffered a different change. In addition to the green hornblende, 

 there have been formed considerable quantities of quartz and not a little 

 brown l)iotite. These rocks have also suffered much more dj-namic meta- 

 raoii^hism than have the eastern ones. They resemble in many resjiects 

 the micaceous hornblendic schists of the Basement Complex, and thus 

 furnish additional evidence in favor of the view that these rocks are 

 squeezed eruptives. 



Of the sheet greenstones, a very few seem to have been surface 

 flows. Others were intrusive sills. Only a few instances are known in 

 which the existence of the latter forms of greenstone may be shown to be 

 probable, although it is believed that many other cases of intrusive sills 

 occur in the Lower ]\Iarquette series. They have escaped detection, how- 

 ever, since their material is similar to that of the dikes, and in a district of 

 complicated .stratigraphy it is almost impossible to distinguish between 

 dikes trending with the strike of the sedimentary beds and flows interleaved 

 with these beds. 



The tuff beds, in the few cases noted, are associated with knobs of 

 nearly massive greenstone. 



Since the material of the pre-Clarksburg greenstones is similar to that 

 of the Clarksburg rocks, and since the former do not occiu- in beds younger 

 than the Clarksburg formation, it is inferred that they are the lower jjortious 

 of the flows, tuffs, and associated greenstones that constitute the Clarksburg 

 formation. 



The post-Clarksburg greenstones comprise only dikes and bosses. 

 The rocks, while more or less altered, are all much fresher than the 

 older gi-eenstones, and all of them have preserved distinct traces of their 

 original structure. These greenstones cut all the rocks of the Basement 



