MAKING THE KOCK BOTTOM. 25 



days bring many children to clamber over it in different 

 places. It appears so black in contrast with the white of 

 the granite that it is probably one of tbe places where 

 Captain John Smith mistook our diabase for slatestone 

 when he came — the first white man — into our harbor in 

 the year 16 14. 



This dike is a little puzzling to classify because it trends 

 so much toward the east as one follows it into the main- 

 land that it seems almost a member of the first system of 

 east-west dikes. 



In that case the part exposed on the side of Little White 

 Head is a wandering crack that missed its direction in- 

 stead of a regular member in good standing of the later 

 north-south system. 



Another north-south dike is worthy of special mention, 

 because of the interesting way in which it cuts a porphy- 

 rite dike at a place near Jerusalem Road a half mile east 

 of Forest Avenue. 



" The diabase dike advances obliquely from the south 

 until it strikes the east wall of the porphyrite dike, follows 

 this wall for twenty-five feet, amputating a branch of the 

 porphyrite dike, and then passes in a graceful double curve 

 diagonally through the latter and follows the west wall as 

 far as either can be traced, — twenty-four feet. The inter- 

 secting dike is a typical example of the third system of 

 diabase dikes — black, brownish-weathering, and beauti- 

 fully cross-jointed." * 



All these dikes along the shore have been kept clear of 

 debris and are easily studied on that account, but these are 

 by no means all of the dikes that have filled the cracks in 

 our granite as the lava was being upheaved. 



How many more there may be inland, covered by the 

 soil, it is hard to conjecture. In fact, they are more apt to 

 be hidden than the granite, for they were more easily worn 



*Quoted from Professor Crosby's Geology of Boston Basin, Vol. I, Part I, Nan- 

 tasket and Cohasset, p. 128. 



