jR. D. Irving — Is there a Huronian Group ? 211 



of the process is recognizable among these ancient quartzites, as 

 well as in the similar rocks of more recent formations. Even 

 in the extent to which this indurating process has been carried 

 on, the Huronian quartzites do not differ so remarkably from 

 many sandstones of much later periods, like which they show 

 also the ripple and other markings so characteristic of water- 

 deposited sand. 



The dark-colored fragmental rocks of this series, which I 

 have embraced under the general head of greywacke, and which 

 vary in coarseness of grain from aphanitic, slaty kinds to 

 rather coarse grits, and are often dotted with pebbles and bowl- 

 ders of granite and gneiss, are, equally with the quartzites, genu- 

 ine clastic rocks, often differing in no respect from similar rocks 

 found in various portions of the world, and at various horizons 

 in the Palasozoic series, where they are found to be frequently 

 highly fossiliferous. Indeed, these Lake Huron greywackes 

 have on the whole undergone much less alteration than many 

 fossiliferous rocks of a similar nature. Those of the north 

 shore of Lake Huron are very regularly stratified, gray to 

 black, often ripple-marked, compact aggregates of rounded to 

 subangular mineral fragments, which include, besides pieces of 

 quartz, particles of various other minerals, particularly of feld- 

 spars. Often the fragments are recognizable as particles of 

 rocks, the disintegration having left the different mineral in- 

 gredients attached to one another. The thin sections of all of 

 these greywackes show that their indurated condition is due 

 partly, perhaps mainly,, to a secondary deposition of interstitial 

 quartz, entirely analogous to that just described as character- 

 istic of the quartzites of the series ; but that it is also due in 

 part to a very general development of chloritic material from 

 the feldspar fragments, to the presence of which chloritic ingre- 

 dient the dark color of the rock is mainly due. 



Limestone is made by Logan to constitute more or less of 

 three numbers of his succession (Nos. 5, 10 and 12 ; respec- 

 tively, 300, 400 and 200 feet in thickness). The lowest one of 

 these three bands, which occurs in the lower half of the series, 

 and is finely exposed along the shore of Lake Huron about 

 two miles west of Bruce Mine Bay, is a thin-bedded, dark gray, 

 often brown-weathering, compact and earthy-looking limestone. 

 The very striking and regular thin lamination, which is 

 brought out with the greatest prominence on weathered sur- 

 faces, the more impure laminae being left projecting in little 

 ridges above the more purely calcareous ones, seems to place 

 the sedimentary origin of this rock beyond a question. This 

 conclusion is borne out very emphatically by the appearance 

 of the thin sections, which show distinctly a fragmental mate- 

 rial mingled with the felted mass of minutely crystalline car- 



