204 THE CRYSTAL FALLS IKON-BEARIXG DISTRICT. 



of sedimentary roek in n dolerite. The rock now possesses the characters 

 of a spilosite, and was evidently l)rought up from below by these intru- 

 sives. No slate is exposed near this point, but it is presumed to underlie 

 this area, although below the exposed volcanics. 



Relations to Lower Huronian Hemlock volcanics. The dolerltC ridgeS whicll OCCUr lu 



the area underlain by the Hemlock volcanics are surrounded on all sides 

 by rocks of related petrographical character. The number of localities 

 at which the relations between the two niaj be observed are very few, 

 but their relations where seen are clear. For instance, in sees. 18 aud 30, 

 T. 44 N., R. 32 W., the coarse intrusives break through the volcanics. lu 

 sec. ?7, T. 46 N., R. 33 W., a boss of the dolerite occurs in the midst of schis- 

 tose volcanic tuffs. The volcanics surrounding the intrusives exhibit sj^mp- 

 toms of more or less violent dynamometamorphic action, whereas the doler- 

 ites in no case show any evidence, microscopically or macroscopically , of hav- 

 ino- undergone the metamorphism from which the volcanics have suffered. 

 The dolerites are thus clearl}' younger than the eifusives of the district. 



Relations to Upper Huronian. — Only a few Isolated doleritc outcrops have been 

 found in the area underlain by the Upper Huronian. The most conspicu- 

 ous outcrops are the large dike in sees. 7, 8, and 9, T. 43 N., R. 32 W., and 

 the knobs in Ts. 42 N. and 43 N., R. 31 W. These last are practically 

 continuous with the great dike which penetrates the Lower Huronian along 

 the ]\lichigamme River immediately to the north. One isolated knob has 

 also been found in the extreme northwestern part of the district in sec. 19, 

 T. 47 N., R. 33 ^Y. Although in none of these areas have the dolerites been 

 found in contact with the Upper Huronian, as it has been shown by the 

 stratigraphy that these areas are underlain by the Upper Huronian sediments, 

 the statement seems warranted that the dolerites are intrusive through them. 



Relations to other intrusives. — In oue placc a dolerite Is inti'uded by a dolerite 

 of later age, and it is highly probable that there are many more similar 

 cases never observed. The dolerites are cut by small granite dikes at sev- 

 eral places east of Mansfield. 



OONTAOT METAMORPHISM OF MANSFIELD SLATES BY THE DOLERITE.' 



The contacts between the dolerites and the sedimentaries are very 

 rarely observable. For the most part where the sedimentaries are altered 



' A oontriljutiou to the study of contact metamorphism, by J. Morgan Clements : Am. Jour, Sci., 

 4tli series, Vol. VII, 1899, pp. 81-91. 



