TUE CLARKSBURG FOKMATION. 465 



'show evidences of the diabasic structure, while many of them aj)])ear to 

 have been porphyritic. 



In the hand specimens these rocks resemble hi many respects the 

 greenstones north of Lake Michigamme (see pp. 500-503). They are prin- 

 cipally massive rocks of a dark-green, almost bhuk color, which varv in 

 grain from very fine to very coarse. On freshly fractured surfaces the 

 finer-grained phases have a more or less fibrous appearance, due to 

 the presence in them of abundant acicular hornblende and plagioclase 

 ciystals. The fresh surfaces of the coarser rocks seem to be made up 

 almost exclusively of large areas of black or dark-green hornblende. 



Under the microscope some of the thin slices of the finer-grained rocks 

 show numerous square or quadi-atic sections of an altered idiomorphic plagio- 

 clase in a groundmass composed of quartz, chlorite, hornblende, calcite, 

 biotite, and a little newly formed feldspar. The quartz and calcite are often 

 present in largest quantity, the former as little interlocking grains, forming 

 a matrix in which the other silicate components lie, and the calcite filling 

 little interstices between these. The amphibole is a yelhjwish-green variety 

 in spicules, and the biotite a reddish-brown variety in small plates. These 

 same minerals are also found embedded in the altered plagioclase, by whose 

 decomposition they were probably formed. All the components of the 

 groundmass of these rocks are apparently new products in their present 

 positions. They were probably derived by secondary processes from a 

 very fine grained or possibly a glassy groundmass of a basaltic porphyrite. 



In other sections there are mottlings of a brown color on a white back- 

 ground when the sections are viewed against white paper. The brown 

 areas contain a great deal of bi< »tite, while the colorless areas are free from 

 this mineral. Tiie latter consist of aggregates of quartz, a little feldspar, 

 and a few hornblende needles, and the former of the same minerals A\'ith an 

 abundance of biotite flakes and large masses of spongv magnetite. The 

 light areas surround the brown ones as the matrix surrounds the jihenocrysts 

 in a porphyrite. 



All the original structures, except the porphyritic, have disappeared 

 from these rocks, so that it is impossible to learn much concerning their 

 MON xxviii 30 



