STRUCTURAL GEOLOGY. 6 



Fragmental rocks of the Archean.] 



weathering. Various chloritic minerals, usually pennine, unite with hornblende and 

 epidote in giving the characteristic colors. Where epidote is abundant the greenness 

 verges toward yellow. Where the chloritic element prevails, the change has probably 

 been due to weathering, and is carried a step further than when hornblende alone 

 gives the green color. Quartz, which is not uncommon in these green clastic rocks, 

 is plainly of two different sources, viz.: of fragmental origin, coeval with the rock, 

 and of secondary origin, the result of alteration of the feldspars. Never in these rocks 

 is there any trace of olivine, so far as observed, but this mineral, which must have 

 been at first embraced in the debris, has been lost by alteration, and its elements 

 divided between chlorite, actinolite and the iron ores. 



As to structure, these greenstones, which, being fragmental, may bear the name 

 greemvackes, are not distinctly stratiform, except in rare instances. They have a close 

 structural relation, as well as a close mineral resemblance, to the igneous greenstones, 

 their chief distinctions being petrographic, and mainly observable in microscopic thin 

 sections. They acquire gradually detrital characters, i. e., they become siliceous, 

 lose their green color, and if fine grained might be denominated phyllyte, and when 

 coarser grained they become graywackes. 



At the same time it is apparent that the volcanic tuffaceous accumulation some- 

 times was accompanied by a copious oceanic precipitation of silica, and occasionally 

 by iron oxide, these usually occurring somewhat sporadically in large quantities, but 

 sometimes very continuously and very widely so as to sensibly change the composi- 

 tion and the color of the resultant rock. This chemical oceanic precipitation is most 

 conspicuous in the fine-grained phases of the rock, the fine green schists sometimes 

 becoming very siliceous and firm, or very siliceous and also reddened by hematite. 

 When, under such a variation, the chemical elements are so abundant as to almost 

 or quite exclude all other ingredients, the resultant rock is that which has been 

 named jaspilyte by Wadsworth.* Frequently, however, the distinctly chemical 

 precipitates are mingled with more or less of the cotemporary volcanic and other 

 debris, and, in rare instances, the jaspilyte is charged with coarse fragmental materials. 



Conglomeratic jaspilyte. A remarkable instance of a jaspilyte containing pebbles 

 and even boulders of granite and other rocks occurs north of Moose lake, near the 

 section line between sees. 20 and 21, T. 64-9. The question of the date and manner 

 of origin of the rock termed jaspilyte was held as a debatable one until the discovery 

 of this curious combination, although, in several of the annual reports, it was claimed 

 to be of the nature of an oceanic precipitate. f Of this locality the following details 

 are important: 



* It is a question whether the term "jaspilyte" should be continued, inasnmch as Wadsworth's idea of the origin of this 

 rock was very different. He described jaspilyte as a rock of igneous origin a surface eruptive of the rhyolitic type. 

 (Compare, specially, Bulletin vi, pp. 55, 103-111. 



