504 ERNEST HOWE 



feldspars seem, in many cases, to have possessed crystal boundaries 

 and were often typically lath-shaped. At present, even in the fresh- 

 est specimens, they are much altered to calcite, muscovite, zoisite, 

 and epidote, and individual grains have been bent and broken, but 

 are not crushed. Hornblende, which is, as a rule, quite fresh, is of 

 the pleochroic fibrous variety, uralite, and there can be little doubt 

 as to its secondary origin; it sometimes appears almost massive, but 

 the borders are extremely ragged, and minute blades and needles 

 are scattered throughout the rock and often penetrate the feldspar 

 areas. Biotite is unimportant and occurs more in the nature of an 

 accessory. 



Although from their mineralogical composition these rocks might 

 be classed as diorites, and although no trace of pyroxene has been 

 found, still from their resemblance to rocks of other regions, especially 

 in the Menominee district, where all the stages in the change from 

 gabbro to hornblende rock miay be observed, they are to be regarded 

 as gabbros in a rather advanced state of alteration. 



Porphyritic and fine-grained greenstones. — The mineralogical com- 

 position of the porphyries and greenstones of finer grain is essen- 

 tially the same as that of the granular rocks, the hornblende being 

 possibly a little more prominent. Alteration, however, is generally 

 more advanced, but strangely enough the original structures are 

 often well preserved, even where the feldspars have been completely 

 saussuritized or changed to other secondary minerals, and the dark 

 silicates have been altered to chlorite. The structure which seems 

 to prevail in all of the finer-grained rocks is the ophitic, so typical of 

 diabase, in which laths of plagioclase lie, as it were in a groundmass 

 of pyroxene and magnetite. In the case of the greenstones the out- 

 lines of the feldspar laths are distinct, and the crystals often appear 

 to radiate from a central point. What should correspond to pyrox- 

 ene is, in the greenstones, either a mesh of uralite needles or chlorite, 

 together with smaller grains of undeterminable feldspar and occa- 

 sionally quartz. Here, there can be no doubt that the rocks were 

 derived from diabase or diabase porphyry by the well- recognized 

 change of pyroxene to hornblende and the alteration of labradorite 

 to saussurite, accompanied by the development of epidote and 

 calcite. 



