1888.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 191 



ON A PERIDOTITE FROM LITTLE DEER ISLE, IN PENOBSCOT BAY, 



MAINE. 



BY GEORGE P. MERRILL. 

 (With Plate xxxiv. ) 



In Dr. C. T. Jackson's Second Annual Beport on the Geology of 

 Maine, 1838, p. 45, there occurs the following passage: 



Neai the southern extremity of the island (Little Deer) we noticed a remarkable 

 mass of greenstone trap, mixed with serpentine, which has burst through the strata 

 of slate rocks and rises to the height of 150 or 200 feet above the sea level. This mass 

 resembles the appearance of a volcano more nearly than any other spot I have seen 

 in Maine. It here protrudes through the slate, which it has torn up all around, and 

 melted in many places into a perfectly white hornstone or chert, while in other places 

 the Chemical action which took place has blown the whole mass into a sort of scoria 

 or amygdaloid. The trap rock is mostly columnar and is broken into quadrilateral 

 columns. A deep ravine separates the slate from the trap, so that it resembles a cone 

 in the midst of a volcanic crater. Several dikes are sent off from the mass through 

 the adjacent rocks. 



Being iu the vicinity of the islaud in the summer of 1887, the present 

 writer took occasion to visit the locality above described and found it 

 of sufficieut interest to merit a more extended notice. Unfortunately, 

 owing to the limited time at our disposal, our party was not able to dis- 

 cover all the points of interest described by Dr. Jackson, many of which 

 have doubtless become more or less obscured during the lapse of nearly 

 half a century since his report was written. 



The mass of "trap" is easily seen from what is known as Deer Isle 

 Landing, on the northwestern extremity of Deer Isle proper, the dis- 

 tance between the two islands being not more than half a mile at this 

 point and the mass itself not more than a mile or possibly a mile and a 

 half distant. From this point it appears in the form of a broad, rounded 

 knoll or boss of a dull reddish-brown color, almost bare of vegetation, 

 and backed by a higher hill of the white " hornctoue " beyond. The knoll 

 itself is locally known as "Pine Hill." On all sides in the immediate 

 vicinity the land is wet and swampy, and covered for the most part by 

 the dense, sometimes almost impenetrable, growth of spruces so charac- 

 teristic of the region. This growth for a distance of several hundred 

 yards from the base of the hill is sufficient to hide every possible con- 

 tact of the erupted mass with the shales, and we could find nothing to 

 indicate that the latter had been torn up and melted in the manner de- 

 scribed by Dr. Jackson. The nearest observable outcrop of the shale 

 was some !200 or 300 yards distant. This will be noticed later. 



The mass of trap rises abruptly, with steeply sloping sides, to a height, 

 presumably, fully equal to that given in the abstract. The rock is 

 eminently massive, very compact, of a deep greenish black color, weather- 

 ing on the immediate surface to brownish, and breaking frequently into 



