148 bulletin: museum of comparative zoology. 



Criteria found: — A, B, C, D, F, H, J, L, M. 



Locality 9. Franklin Field. At the junction of Blue Hill Avenue 

 and Harvard Street there is an outcrop of tiUite. No strike or dip are 

 available. The breadth of the outcrop is about 100 feet. The 

 matrix is a sandy slate. The included rock fragments are angular, 

 subangular, and rounded. One striated pebble has been found. This 

 pebble is shown in Plate 10. Blunted and bevelled stones are common. 

 Several concave fractures have been found. The rock fragments are 

 composed of granite, felsite, and quartzite. I have not found mela- 

 phyre, and there are no slate masses. There are no intercalated 

 layers. The transition-bed or beds to the Roxbury formation are 

 conglomerate, and not slate or sandstone as in localities farther south 

 and southeast. Shearing was as intense at this locality as at any other. 



Criteria found: — A, B, C, D, E, F, G, H, J, K, L. 



Locality 10. Atlantic. About one half of a mile southwest of the 

 aviation field at Atlantic, on a wooded knoll, a very important expo- 

 sure of the tillite may be seen. The series of beds here commences 

 well down in the Roxbury conglomerate proper, and in almost con- 

 tinuous outcrops ends near the middle of the tillite. This is the best 

 exposure of the beds underlying the tillite, with the tillite bed itself 

 well exposed. Commencing at the most northern extremity of the 

 knoll the Roxbury has a strike of N 42° E, with a dip of 45° S. The 

 transition-beds below the tillite have a strike of N 48° E, and a dip 

 of 70° S. Thicknesses of beds in this section are as follows : — Rox- 

 bury conglomerate 520 feet; a sandstone bed twenty-five feet; con- 

 glomerate, sandstone, and slate 120 feet; contorted slate and sand- 

 stone forty-seven feet; conglomerate and sandstone 123 feet; tillite 

 298 feet. This gives a total for the section of 1,133 feet. In correct- 

 ing the thickness of the Ro.xbury conglomerate an average dip of 57° 

 was used, the dip at the bottom being 45° and at the contact with the 

 first bed of sandstone 70°. 



At this point it is necessary to describe some of the beds underlying 

 the tillite, for the reason that two of these bqds resemble a bed in 

 Brighton. About fifty feet abo\'e the main body of the Roxbury 

 formation, and just above a sandstone layer about 3 feet thick, there 

 appears a bed of conglomerate with thin layers of slate and fragments 

 of slate. These fragments are very irregular in shape and vary in 

 size, although most of them are not over six inches in diameter. It is 

 evident that this deposit is water laid, and that the fragments have 

 been washed along with the pebbles. In view of the fact that much 

 larger fragments of slate are found in the tillite at Squantum and 



