TIIK CLARKSBURG FORMATION. 467 



mineral is of rluj sainc nature a.s tliat in the other <^Teeustones, but is in 

 much larger (![uautity. It occurs as compact anhedra,' fringed with long 

 acicular crystals that form a network, in whoso meshes are areas of" altered 

 plagioclase and leucoxene. 



From the above rapid survey of the coarse greenstones in the Clarks- 

 burg series it will be seen that these rocks do not differ essentially from 

 the "greenstones" subsequently described as intrusive in the pre-Clarksburg 

 beds. They are strikingly similar to those in the western portion of the 

 district (see jjp. 499-50(i). They were originally of the same composition as 

 these, and they have suffered similar alterations. Their structure, where 

 it can be detected, was a little different from that of the lower intrusive 

 greenstones in that there was a tendency to the production of poqjhyritic 

 feldspars. This difference may be due to the fact that the Clarksburg rocks 

 were cooled in the ducts of volcanoes or in dike fissures that were near the 

 surface, Avhile the lower greenstones were cooled at greater depths. In this 

 connection it is interesting to note that rocks like these gTeenstones are not 

 found intrusive in horizons higher than the Clarksburg. 



THE LAVAS. 



The lavas that are interbedded with the sediments and tuffs of the 

 formation are not very abundant. Some of the finer-grained greenstones 

 already described jnay be portions of lava flows, but that this is certainly 

 the case has by no means been shown. A few layers identical with the 

 former in structure and composition are unquestionably sheets. 



The clearest e\'idence that genuine lava flows wei'e laid down among 

 the more abundant tuft' beds of the old Clarksburg volcano is afforded by the 

 amygdaloids. These have been found in a few widely separated localities 

 within the limits of the Clarksburg belt, but they are not at all common. 

 The rocks are fine-grained, light-gray, and often schistose. They contain 

 few or many amvgdules filled with calcite or chlorite, and these are often 

 flattened as though b}' flowage. Under the microscope they present no 

 peculiar features. Small laths of plagioclase, with forked extremities, are 



'This term has recently been propo.sed by Pirssou to designate those crystalline constituents of 

 rocks that do not possess crystal outlines. Bull. Geol. Soc. Am., vol. 7, 1896, p. 492. 



