THE LOWER HURONIAN. 285 



found occasionally, but they are mucli rarer than one would expect them to 

 be. In addition to the kinds of rock already enumerated, pebbles of black 

 and gray chert and yellowish-green sericite-schists were observed. 



The conglomerates differ locally in degree of coarseness, varying from 

 coarse-grained conglomerates, with bowlders reaching 2 feet in diameter, 

 to those in which the majority of pebbles are about 4 to 5 inches in 

 diameter. This latter facies is the commoner. Associated with these 

 conglomerates there are of com^se considerable quantities of much finer- 

 grained rocks, which would naturally be called consolidated grits or gray- 

 wackes, but which are here mapped with the conglomerates. With these 

 are likewise occasionally areas of slate. On the map an attempt has been 

 made to discriminate, by means of the colors, between the conglomerate 

 and the slate, but a close examination in the field would reveal the fact 

 that in some of the areas marked as conglomerate there are in places 

 considerable quantities of graywackes and slates associated with and lying 

 in the midst of the conglomerate. The areas of these rocks ai-e so small 

 in proportion to the area of the conglomerates that no attempt has been 

 made to show them on the small scale maps published herewith. 



It is interesting to note the dependence of the petrographic character 

 of the conglomerate upon the adjacent rocks from which it was derived. 

 Where, for example, it lies next to a certain characteristic porphyry, 

 the major portion of the conglomerate is formed of pebbles and fine detritus 

 of the porphyry. On the other hand, where the conglomerate lies near 

 the u'on-bearing formation, fragments of this formation become numerous, 

 although ordinarily they are scarce. The pebbles and bowlders in the 

 conglomerate are crossed by fracture lines which divide the individual 

 pebbles in it into more or less rhomboidal fragments. This fracturing of 

 the fragments and the occurrence of the pieces essentially in place shows 

 that the dynamic action that produced the fracturing took place after the 

 formation of the conglomerate and that only slight displacements occurred 

 as the result thereof. 



ORIGIN OF THE CONGLOMERATES. 



When we study these conglomerates in the field and find that they 

 are made up of pebbles of various kinds of rock lying in a fine-grained 

 clastic matrix, the pebbles of a certain kind of rock being most numerous 



