^> 



64 



BULLETIN" OF THE 



bles, as well as the deposition of the silica, do not occur where the 

 sandstone has been seen to come in contact with the "Iluronian'* 

 schists and quartzites. From the above facts we feel that we arc justi- 

 fied in dissenting from the views of most of those who have written 

 upon this locality. 



At the eastern portion of Presque Isle, either a fault or a protrusion of 

 the peridotite up through the sandstone exists. This locality we were 

 able to examine only a minute or two, on account of an approaching 

 thunder-storm, while we were in a row-boat, but it deserves further care- 

 ful examination. The best serpentine that we saw upon Presque Isle was 

 observed at this place. In the thin section, the more serpcntinons 

 portions of the peridotite are frequently seen to contain dolomite, and 

 fragments effervesce freely in hot iiydrochloric acid. The upper portion, 

 supposed to be dolomite by Rominger, extends as a sheet of variable 

 thickness over all the peridotite separating it from the overlying sand- 

 stone. This, so far as we can tell, is the upper portion of the peridotite, af- 

 fected by direct contact with the sandstone, and by the action of hot waters 

 since, at the time the silica was deposited in the conglomerate. This 

 part is fdled in with impure dolomitic veins, which, it seems, caused Dr. 

 Rominger to pronounce the rock a dolomite, although the veins have 

 the characters of being secondary water deposits, as they were described 

 by Messrs. Foster and Whitney, and not igneous injections, as they were 

 thought to be by Dr. Houghton. The unmistakable peridotite has in 

 places the same structure. The peridotite is much fissured, breaking up 

 into rounded masses, cemented by segregated serpentine. We regard, 

 the peridotite as eruptive, and feel that its field and microscopic charac- 

 ters both point to the same conclusion. The manner of eruption was 

 probably something like the laccolites described by Mr. G. K. Gilbert; 

 on the sides the strata were arched and bent upwards, but on the eastern 

 end either a fault or protrusion exists. This place would afford an 

 excellent chance to study the relations of the sandstone to the peridotite, 

 if the contact of the two can be seen. 



If we are right in our observations and conclusions, this locality has 

 an important bearing upon the origin of the serpentine here, showing 

 that it is a metamorphosed eruptive rock, and of younger age than at 

 least some one hundred feet of the sandstone. It further shows that, 

 so far as this is concerned, lithology fails in giving the age of a rock, 

 this having been indorsed as good '^ Huronian" ;* also that an eruptive 



* Mr. Brooks thinks "it is not certain that this is of Huronian age." (Gool. of 

 Wise, III. 659.) 



