ii6 Bulletin of Laboratories of Denison University. [Voi. xii 



many instances the thin-sections show both stout and acicular 

 forms of striated feldspar partially or wholly preserved, em- 

 bedded in a fine-grained groundmass composed principally of 

 green chlorite and hornblende, epidote, altered feldspar and iron 

 oxide. This arrangement is not confined to the massive and 

 least altered forms of the rocks, but is indicated to some degree 

 in the partial skeleton outlines of some of the original minerals 

 in several slides of the perfectly schistose rocks. In many cases 

 chemical and structural metamorphism have progressed so far 

 that all trace of the original structure, as well as that of every 

 original mineral, has been destroyed. 



The occurrence of lath-shaped polysynthetically twinned 

 crystals of plagioclase which appears to have formed an essen- 

 tial constituent of the rocks is characteristic of rocks of igneous 

 origin. Furthermore, the microphitic and poikilitic structures 

 of the feldspars of some of the rocks in thin-section under the 

 microscope are common only to igneous masses. The struc- 

 tures bear certain striking resemblances to similar rocks of 

 igneous origin described by Williams^ and Clements^ from the 

 Lake Superior region. Professor Williams^ reproduces a pho- 

 tomicrograph of a thin-section of one of the rocks showing this 

 structure from the Negaunee district, which has its analogue in 

 several sections of the Virgilina rocks. 



The minerals composing the rocks, which are chiefly sec- 

 ondary, are those which would result from chemical and struc- 

 tural metamorphism of an original igneous rock of basic or in- 

 termediate composition. 



While, as already stated, in most instances all trace of the 

 original minerals in the rocks is lost or destroyed, in some sec- 

 tions enough remains to tell with some degree of certainty what 

 their original essential minerals were. 



The analyses of these rocks given in the table on page 1 1 2 



> Williams, G. H., Bulletin No. 62, U. S. Geol. Survey, 1890. 



* Clements, J. Morgan, Jour, of Geology, 1895, Vol. Ill, pp. 801-822; 

 Monograph, No. XXXVI, U. S. Geo!. Survey, pp. 98-103. 



' Williams, G. H., op. cit., p. 226, plate X, figure 2. 



