AMPHIBOLITE. 51 



These two rocks, gneiss and amphibolite, constitute the main mass of 

 the Archean schists, mica schists, phyllite, and other thinly bedded rocks not 

 occurring in any well defined bodies. Peculiar schistose forms do appear 

 in the gneissic series, but are subordinate in every respect, with only local 

 extension, and of abnormal constitution. In the contorted state of the strata, 

 the tracing out of the relations of these bodies to the gneiss, while extremely 

 interesting, would have taken much more time than could have been devoted 

 to this subject. A few examples will show the interesting nature of these 

 masses. 



On the north face of Mount Lincoln occurs a contorted schist of dark 

 color, in which the naked eye can determine biotite and small flakes of 

 glistening muscovite. The microscope shows that the two micas form 

 nearly the whole rock, the compact appearance being due to extremely 

 minute flakes of biotite, often so small as to require a power of 800 diame- 

 ters to distinguish them clearly. Between these two elements, in varying 

 quantity, is a mass appearing between crossed nicols like the decomposition 

 product of orthoclase in many of the older rocks, where muscovite in tiny 

 flakes has been the chief mineral formed ; this substance is here very uni- 

 form in composition, giving the brilliant polarization colors of such an 

 aggregate, and, as no feldspathic substances can be detected, it remains 

 uncertain whether this muscovite comes from orthoclase or is original, cor- 

 responding to the minute leaflets of biotite. No hornblende is visible. 

 Tourmaline in bundles and brushes is the next most abundant element, 

 being brown in ordinary light, with a tinge of red or blue; a few small 

 grains of quartz, and specks of ilmenite altering into "leucoxene," are the 

 only remaining minerals. 



RELATIVE AGE. 



The Archean rocks just described are all without question older than 

 any of the Paleozoic series, which rest unconformably upon them; but 

 of the relative age of these different components of the ancient crystal- 

 line series it is in the nature of things difficult to form any very decided 

 judgment. Even had time permitted a careful and detailed study of any 

 of the remarkable exposures in the great glacial amphitheaters which have 

 been carved out of them, it is doubtful whether their original relations 



