THE ACADIAN TRIASSIC 261 



FIVE ISLANDS VOLCANICS 



Under the heading Five Islands volcanics are included the tuffs, 

 agglomerates, and basalt flows in the vicinity of Swan Creek and 

 the Five Islands. The thickness of the volcanics is estimated as 

 at least 350-400 feet. One associated dike is exposed at Gerrish 

 Mountain. 



The Gerrish Mountain diabase dike is almost vertical and 

 about 20 feet or more in thickness. The diabase shows marked 

 columnar jointing, the columns being rather short and largely 

 horizontal or dipping at a low angle to the horizontal. The dike 

 is connected with the basalt flow which caps the sandstones of 

 Gerrish Mountain, and it has evidently furnished the material for 

 this flow and perhaps for a large part of the other igneous rocks 

 for the vicinity. 



The basalt flows associated with the Five Islands volcanics 

 are found at Gerrish Mountain, on four of the Five Islands, on 

 Two Islands, and at Portapique Mountain (east of Gerrish Moun- 

 tain). It is noteworthy that the relation of these flows to the 

 agglomerates is unknown, and that there is no proof that they are 

 not connected with the North Mountain basalt instead of with the 

 Five Islands volcanics. The structure of these flows is in large 

 part columnar, and the base and the top of each individual flow is 

 marked by amygdaloid. The basalts are the usual fine-grained, 

 dark-gray, heavy rocks composed of augite and plagioclase with 

 accessory amounts of magnetite and occasionally olivine. A more 

 detailed petrographical description will be given below for the North 

 Mountain basalt, which will apply equally well to these flows. 



Only one flow is exposed in Gerrish Mountain. This has a 

 thickness of over 75 feet. Three flows are exposed on the north 

 side of Moose Island, the upper one being agglomeratic. A portion 

 of a single flow is exposed on Diamond Island and on Long Island. 

 Two flows are seen on Pinnacle Island. The northern of the Two 

 Islands consists of three flows, the southern of probably only one. 



The base of the series of flows is exposed on the eastern side of 

 Moose Island and on Gerrish Mountain. Under the amygdaloid 

 which marks the base of the flow is a layer of green ash 2-3 feet in 



