210 jR. D. Irving — Is there a Huronian Growp f 



The thicknesses of the interbedded eruptives are not sepa- 

 rately given, but their subtraction would not seriously affect 

 these figures, except in the case of division No. 2, which, as 

 subsequently shown, is very largely composed of diabase sheets. 

 Placing together the several kinds of material composing- 

 Logan's section, without reference to stratigraphical order and 

 without making any correction for included eruptives, the 

 series is seen to be composed as follows : quartzite, 10,820 feet ; 

 "slate conglomerate," 4280 feet; "chloritic and epidotic slates 

 and trap" 2000 feet: limestone and chert, 900 feet=18,000 

 feet. But a much more accurate conception of the relative 

 proportions of the different kinds of rocks which constitute 

 this series may be reached by making several important modi- 

 fications of this statement. In the first place my own exam- 

 inations of the ground, while they have served to convince me 

 of the general correctness of Logan's section, have shown me 

 that considerable portions of his " slate conglomerate " mem- 

 bers are made up of quartzites, the remainder of these members 

 being composed of dark-colored fragmental rocks, to which the 

 name of greywacke more nearly applies than any other given 

 in the books. Moreover, that portion of the series designated 

 by Logan as "chloritic slates and traps" (No. 2), I have found 

 by a study on the ground, and of a number of thin sections, to 

 be mainly composed of a succession of diabase-sheets, along with 

 which is a little interleaved fragmental material, perhaps partly 

 of the nature of volcanic ash, but with which there is also some 

 true sedimentary material. Taking these facts into account, 

 and remembering also the interleaving of greenstone sheets at 

 different, horizons in the series, I estimate the whole succession 

 to be about two-thirds quartzite, one-sixth greywacke, and one- 

 twentieth " lime-stone and chert," the remainder being chiefly 

 composed of eruptive material. 



Of these rocks the quartzites are all no more nor less than 

 indurated sandstones, without prominent schistose structure. 

 The induration — which varies so greatly in degree that some 

 of these rocks are almost loose sandstones, while others are com- 

 pletely vitrified quartzites, the two extremes occurring at times 

 in close association with one another — is always due to the 

 presence of an infiltrated silica. This infiltrated material occurs 

 partly in the shape of a minute mosaic between the plainly 

 recognizable rolled fragments of quartz, of which the rock is 

 mainly composed ; but is for the most part in the shape of 

 enlargements or secondary growths to these fragments. This 

 process of enlargement, which I have described and illustrated 

 freely elsewhere, is one which, as I have been able to show, has 

 affected fragmental quartzose rocks of all ages, from the most 

 recent to the most ancient. Every stage in the development 



