4 PROCEEDINGS OF THE ACADEMY OF [Jan., 



penetrated by several dikes and intrusions of igneous rocks. A 

 series of these rocks was collected and this particular place selected 

 for the purpose, not only because the intrusions seemed to include the 

 principal types of igneous rocks, but also for the reason that they are 

 located where they can be readily identified from the description by 

 anyone interested. 



Commencing with the rocky point which extends out to low-water 

 mark on the north of the bathing beach, this is penetrated by a dike 

 about twelve feet thick of diabase porphyrite with phenocrysts of 

 plagioclase too much zoizitised for specific identification, in a matrix 

 of diabasic texture, composed of augite, biotite, plagioclase, and 

 chlorite, the latter apparently altered pyroxene; also as accessory 

 constituents, titanite, apatite, and secondary calcite. Dr. F. Bascom, 

 who kindly looked over these sections wath me, suggests that the 

 reason much of the pyroxene is entirely fresh or in part altered to 

 hornblende, while in other cases it is completely replaced by chlorite, 

 is probably that there may have been two distinct varieties of pyrox- 

 ene originally present, one more readily altered than the other. 

 Near contact with the shale, this dike becomes basaltic in texture, a 

 fine-grained mixture of feldspar, biotite, magnetite, and bro^vn horn- 

 blende, the latter no doubt replacing primary pyroxene, with pheno- 

 crysts having the outlines of pyroxene, almost invariably completely 

 altered to chlorite. 



A short distance toward the south, in the rocky wall back of the 

 beach, is a twelve-inch dike of diabase with a small branch dike 

 forking from it. Except that it contains a few small vesicles filled 

 with secondary calcite, this is a typical diabase, fine and uniformly 

 grained. Beyond it is a dike of basalt, four to eight inches thick. 

 At the contact it is glassy, with lath-shaped feldspars oriented parallel 

 to the wall. The interior is more completely crystalline, with 

 phenocrysts of pyroxene altered to chlorite and many small, rounded 

 patches of calcite, apparently filling vesicles. 



Further south is an irregular angular intrusion of trachyte. It 

 consists almost exclusively of intermeshed rods of feldspar, apparently 

 orthoclase somewhat kaolinized, with scattered patches of ferru- 

 ginous material slightly translucent and dark red in color when 

 sufficiently thin, also generally red by reflected light. This rock 

 corresponds in texture to the dyke rocks which have received the 

 name of bostonite, but in the absence of any microscopical evidence 

 of the presence of anorthoclase, a chemical analysis would probably 

 be necessar}' to determine whether it should be so classed. 



