308 THE JOURNAL OF GEOLOGY. 



usually tilted at high angles. There are numerous instances where there is a 

 gradual transition from the Huronion to the lower series. A few instances of 

 local want of conformity between the two is no evidence that the two systems 

 are not conformable on a grand scale. The few known instances where there 

 appears to be a want of parallelism are more probably due to faulting. The 

 pyroclastic rocks show the agency of water in their formation, and were largely 

 derived from igneous matter, which had been more or less recently erupted. 

 The newest rock of the Sudbury district is a volcanic breccia, wh-ich forms a 

 continuous range of hills for a distance of thirty-six miles, with a breadth in 

 the center of eight miles. Within the Huronian rocks are intrusive red granites. 



Comments. Attention is called to the implication that the unconformity at 

 the base of the Huronian, if it exist at all, is of a local character. The very 

 idea of an unconformity pre-supposes that it can not be local in the narrow 

 sense. A minor unconformity even marks a considerable time break, and 

 when an earlier series has been profoundly metamorphosed and deeply 

 denuded before the overlying series is deposited upon it, the break must be 

 of regional extent, even if the contacts found are few and of small extent. It, 

 however, does not follow that the break is universal nor even that it always 

 extends throughout a geological basin. Space does not permit a discussion 

 of the evidence for the existence of unconformable contacts at the base of 

 the original Huronian in certain localities. It is enough to say that Irving, 

 Pumpelly, Reusch, Barrois, and Tschernychew, all having seen one of the 

 localities and the first two both, agree that the only interpretation of the 

 phenomena at points near Garden river and near Thessalon is that of a great 

 unconformity, not faulting as suggested by Bell, who does not appear to have 

 ever visited these localities. 



Barlow ' states that the Huronian system is the oldest sedimentary strata 

 of the north shore of Lake Huron, and that the Laurentian gneiss or Base- 

 ment Complex is the original crust of the earth or floor on which the first 

 sediments were laid down. This floor, as shown by the pebbles of the 

 Huronian, was granite which had in many places a foliated or gneissic struc- 

 ture. In many places the subsequent folding and fracturing of the compara- 

 tively thin crust of the earth has caused large portions of the Huronian to 

 sink below the plane of fusion, the result of which has been to produce 

 irruptive contacts. At other places, as described by Pumpelly and Van Hise, 

 the Basement Complex may have remained undisturbed so that the overlying 

 detritals have not been intruded by the granitic mass beneath. 



Hall and Sardeson"" describe the Upper Cambrian rocks of Southeastern 



'^ On the relatio7is of the Laurentian and Huronian on the North Side of Lake 

 Htiron. Alfred E. Barlow. Am. Jour. Sci., 3d series, vol. 44, 1892, pp. 236-239. 



= Paleozoic Formations of Southeastern Minnesota. C. W. Hall and F. W. Sardeson. 

 Bulletin Geol. Soc. of America, vol. 3, 1892, pp. 331-368. 



