THE EASTERN AKEA. 377 



nizable minerals contained in tlie giassv varieties are tabular plaj^ioclases, 

 1-eucoxene, and titanite, ehlorite, and epidote. The luilocrystalline variety of 

 ))ebble is sometimes fresh enou<ih to be distinctly recognized as a diabase- 

 |)or[»hvrite: the more altered ones would 1)e classed as porjdivrites. 

 These have as chief constituents the miner;ds iil)ove mentioned with 

 the addition of actinolite or hornblende, and occasionalK' augite and 

 nicnaccanite, with almost always more or less of secondary (juartz. '^Plie 

 chlorite and ejjidote are in large measure the result of the alteration of an 

 original pyroxene mineral, although in part tliey are derived from the 

 decomposition of feldspar, ami apparently the actinolite comes from the 

 siimc sources. The (juartz is believed to be always secondary. AVhen tlie 

 pebbles are of an amygdaloidal character, the amygdules are usually of 

 (piartz or calcite or ej)idote, or two or all three of these combined. These 

 augite-porplivrites and porphvrites resemble closely the rocks described bv 

 Irving as diabase-porphyrite and ash-bed diabase in the Keweenaw series.' 



The second variety of pebbles, the red felsitic-looking kind, differs 

 only tVom those just described in that the constituents are deeply stained 

 with oxide of iron. They are then simply feiTuginous porphyrites. It thus 

 appears tliat almost all of the larger fragments of the gTeenstone-conglom- 

 erates are in their character basic eruptives. Resides the fragments thus 

 derived from basic eruptives there are rarely found well rounded pebbles 

 of white quartz. These pebbles, although so infretjuent, are important as 

 bearing upon the probable oiigin of the rocks. In one case there is in a 

 section a roumled (juartz area which is complex, and which is composed of 

 simple (piartz grains now interlocking l)y enlai'gement. This pebble was 

 then derived i'rom a sandstone, which was changed to a (piartzite either 

 prior to oi- since its deposition in its present place. Aside from tlie classes 

 of fragments mentioned there is not infrequently jjresent in the matrix 

 large simple well rounded grains of ((uartz and fehlspar. These ipiailz 

 grains are often enlarged. Tlun', like the ([uartz-pebbles just referred to 

 would seem to indicate that the rock is water-deposited. 



Oriji'm of Ihr Grefinstoiir-cni/f/loiiu'nifi's. — The one feature in conunon 

 which nearl\' all exposures of the area (dassed under this teriu have is the 



•■R. D. Irviug: Copper-bearing rocks of lake Superior; Mouograph U. S. Geol. Survey, vol. v, 



1883, pp. 77-87. 



