96 bulletin: museum of comparative zoology. 



them. This dike crosses the road in a direction X. 70'' E. (magnetic) 

 having a width at the road of 23 meters (75 feet). The sandstones 

 on the north side of the dike strike north-south (magnetic) and dip 

 ca. 0° west. The sandstone on the south side of the dike strikes 

 N. 23° W. (magnetic) and dips 7"" west. West of the road the dike is 

 traceable by means of small weather-boulders in a brook for about 1,098 

 meters (3,600 ft.) but the trace of the dike to the east of the road was 

 not followed. (See Plate 18.) 



In the disintegrated rock of the dike there are crystals as large as a 

 hen's egg of a now rusty black augite and large flakes of a black mica 

 carrying holes from which some included mineral of earlier genesis 

 has been dissolved out. A soft, white, partly altered, mica is probably 

 bleached biotite. The feldspar constituent was not observed. 



The most striking feature of this dike is the large number of in- 

 clusions of foreign rocks which it contains. These constitute at the 

 exposure in the road quite one half of the \-olume of the dike and 

 comprise at least the following varieties of older rocks, viz: — 



Red sandy shale in fragments up to 51 cm. (20 inches) diameter. 



Red shale; also a black shale. 



Coarse grained basalt with lathe-shaped feldspars. 



Fine grained basic rock. 



Amygdaloidal basalt. 



Gre3'ish amygdaloidal basalt, less vesicular la\ a than the preceding. , 



A fine-grained, dark, thin-laminated rock weathering white, origin 

 not determined. 



The fragments of sedimentary rock may well be regarded as dis- 

 rupted from the walls of the fissure which appears to have been the 

 locus of a \ertical displacement of the strata. Presumably the 

 fragments came from underlying strata though they may just as well 

 have fallen in from above. The vesicular lavas of different types, 

 unless there are flows buried beneath the Lages area of sandstones, 

 contrary to my own determinations of the geological structure and 

 those of Dr. I. C. White, must have been derived from overlying lavas. 

 Though rarely, amygdales form vertical bands in dikes, they are 

 characteristic of lava-flows or of lava which has been raised to the 

 vent of a volcanic conduit. This dike apparently communicated in 

 its time with the surface and permitted lavas already extravasated 

 and cooled to yield fragments which sank in the still fluid magma of 

 the dike. So much of the history which appears in the nature of the 

 phenomena leads to the further conclusion that the dike communicated 

 at the time of the infalling fragments with the Corisco, or some vet 



