370 



The details of the geological structures of the region 

 are largely unknown. The Pre-Cambrian strata are 

 presumably much faulted and folded. They have been 

 intruded by large and small bodies of plutonic rocks. 

 Different broad divisions of the Pre-Cambrian are developed 

 along northeasterly trending axial lines and this mode of 

 distribution doubtless indicates that the axes of folding 

 and the strikes of the major faults are also parallel with 

 the same general direction. The Cambrian and Carboni- 

 ferous measures are also folded along northeasterly trending 

 axes, but, whereas the Cambrian beds in places are tightly 

 folded and overturned, the Carboniferous measures lie 

 in open folds. 



The Pre-Cambrian strata include both sedimentary and 

 volcanic types probably belonging to groups of greatly 

 differing ages. So far as we know, all have been intruded 

 by plutonic masses varying in composition from gabbro to 

 granite and not all of the same age. Possibly some of the 

 plutonic rocks classed with the Pre-Cambrian may be of 

 Palaeozoic age and the same may be true of some of the 

 volcanic rocks and even of some of the metamorphosed 

 sediments. The Pre-Cambrian rocks have been divided 

 into various divisions grouped under the terms Laurentian 

 and Huronian. The Laurentian has been described as 

 characteristically composed of crystalline limestone, 

 quartzite, various schists, gneisses and granitic rocks. 

 The Huronian has been defined as composed of great 

 thicknesses of volcanic rocks including flows and pyroclastic 

 types. The correlation of the volcanic strata with the 

 Huronian as now defined, cannot be upheld. The corre- 

 lation of the crystalline limestones and associated strata 

 with the Laurentian is also of doubtful value. 



The Pre-Cambrian rocks were greatly deformed, intruded 

 by plutonic bodies and deeply eroded certainly earlier than 

 Middle Cambrian time and in all probability earlier than 

 Lower Cambrian time. 



The Cambrian and Ordovician strata have been divided 

 by Dr. Matthew into a number of divisions. These are 

 presented in the following table based on one prepared by 

 Matthew. 



