32 bulleti:n" of the natural histoky society. 



can only be assumed from their association and tlie find- 

 ing of intermediate stages of alteration. 



Basic Volcanic Bocks. — A more basic type of volcanie 

 rocks is to be seen in a series of knobs along the southern- 

 side of the Cambrian slates. The heights on which the 

 Martello tower is built are of this material, and to the 

 east of the city it crops out on the shore south of the 

 alms-house, and again back of the penitentiary. This 

 rock is a plagioclase-augite mixture, or diabase — one of 

 the rocks commonly called " trap." It is often very full 

 of vesicles, which are filled in with white Cjuartz. At 

 the shore, by the alms-house, it outcrops as a heavy 

 dyke in green and red slates, instead of a surface 

 flow ; in all j^robability part of the fissure is exposed, 

 through which the magma found its way to the surface. 

 It is interesting to note that as far back as 1840 Dr. 

 Gesner pronounced this to be a dyke, though he was not 

 able to find its edges, and supposed it to graduate into 

 the surrounding slates. By a careful examination, how^ 

 ever, the edges may be traced on both sides ; but with 

 some difiiculty, because the igneous rock has baked the 

 slate near by into a hard flinty mass, very like itself. 



Dykes. — Great numbers of small dykes are to be 

 found cutting across all the older rocks near St. John. 

 Most of them, probably, were formed about the same 

 time as the great volcanic surface flows, which they 

 closely resemble, though wanting the special characters 

 which have been mentioned as distinguishing surface 

 flows. Almost all are diabase, dense, sometimes porphy- 

 ritic, more generally fine grained. A very few porphyry 

 and porphyrite dykes are also known. 



