241 



perhaps five hundred feet north of Boylston Street, exactly the 

 same slate, though somewhat contorted as well as ripple- 

 marked, dipping N. 50"^ W. 38°. At both points this slate is 

 very fissile in the direction of the bedding, and near the river 

 it holds an occasional stray pebble. About two hundred and 

 fifty yards north of the exposure on the river bank there is a 

 more prominent ledge of slate ; this rock is very fine, showing 

 no trace of ripple marks or of an arenaceous or pebbly texture, 

 and is of a pure gray color. It is very thin-bedded, and much 

 crinkled. The dip is N. 45° W. 33°. On the southern end 

 of this ledge the slate is cut off along the strike by amygdaloid. 



North and west of these slate ledges there is a wide area of 

 unbroken drift, and the overlying rocks cannot be seen. On 

 the west side of the river there is no slate exposed ; but on the 

 line of strike of the slate belt, north of Boylston Street, there 

 is first conglomerate, with arenaceous beds, dipping to the 

 north, then a large, high mass of amygdaloid (probably con- 

 nected with that lying to the north-east on the east side of the 

 river, and mentioned in the last paragraph), which is followed 

 by more conglomerate in obscure ledges not showing dip ; and 

 beyond this, both north and west, everything is hidden for 

 nearly a mile. South of Boylston Street, west of the river, 

 the conglomerate outcrops abundantly and in large masses ; 

 and where it adjoins the amygdaloid on the west the relations 

 of the two rocks are well displayed. The conglomerate is 

 thoroughly consolidated and indurated near the contact, and 

 yet it is largely composed of pebbles of apparently the same 

 amygdaloid with which it is so curiously involved. The contact 

 is extremely irregular, and portions of the conglomerate appear 

 to be isolated from the main mass. 



More than one observer has appealed to the Upper Falls and 

 Chestnut Hill band of slate as proving conclusively that there 

 is at leagt one considerable body of slate in the Boston basin 

 underlying, in its riormal position, a portion, at least, of the 

 conglomerate. That this slate overlies the conglomerate on 

 the south can scarcely be questioned, and is generally admitted. 



OCCAS. PAPERS B. S. N. H. — III. 16 



