THE LOWER HURONIAN. 289 



line, each fing-er pointing to a small fold — anticline or syncline. Tlie plane 

 of contact between the porphyry and the sediments varies greatly. In 

 most cases the porphyry is below the sediments, but in some cases the 

 fold is clearly overturned, so that now the conglomerate frequently 

 lies under the porphyry. Between these extremes any position of this 

 contact plane, from flat to vertical, may be seen. This irregularity 

 in the position of the plane caused considerable confusion at first in 

 the determination of the relationship between the rocks. For some time 

 it was thought that the porphyry was intrusive in the sediments. How- 

 ever, further study showed that the congloiuerate was clearly derived 

 from the porphyry and that the relationship mentioned was due to close 

 folding. A further factor which led to confusion was that the porphyry 

 itself simulated somewhat a conglomerate, for it is marked by two series 

 of fracture lines lying close together and crossing each other at such an 

 angle as to produce small rhomboidal blocks. Further shearing took place 

 along these planes of parting, and eventually the angles of the fragments 

 were more or less completely rounded, and the areas between the sub- 

 angular fragments were filled with schistose material. On exposed sur- 

 faces the massive unfractured parts of such rocks weather less readily 

 than do the schistose portions lying between them, and consequently 

 stand out as small rounded projecting areas very similar to the pebbles 

 in a conglomerate, which project above the surrounding matrix. Closer 

 examination of such surfaces, however, shows that the fragments are all 

 of one kind of rock and that the apparent matrix lying between them is 

 but sheared material of essentially the same nature as the massive portion. 

 This is the most obvious fact noticed in a study of them and enables one 

 readily to separate such fractured and sheared porphyries from the true 

 conglomerates derived from them, which are made up invariably of frag- 

 ments of diiferent kinds of porphyries, with more or less abundant jasper 

 fragments and an occasional fragment of greenstone. The strike of the 

 axes of the main folds at th's locality is about N. 80° E., showing that 

 the force that produced the folding was exerted along a line extending 

 approximately north and south. As the result of this compression schistosity 

 has been developed in the sediments and in the underlying intrusives. 

 This schistosity cuts directly across the minor folds shown in the zigzag 

 contacts above described and continues from the sediments into and 



MON XLV — 03 19 



