BURR: MELAPHYR OF BROOKLINE, BRICxHTON, AND NEWTON. 67 



strikes N. 15° E. and dips west. On the opposite side of the street the 

 melaphyr rises abruptly and extends westward in a prominent ridge to 

 Foster Street and continues to Lake Street. Thus the conglomerate 

 dips directly beneath the end of the melaphyr ridge and strikes across 

 its trend. Considering this in connection with the attitude of the rocks 

 on Foster Street, it seems probable that the melaphyr at this point 

 actually cuts the sediments across their strike and is thus truly a 

 great dike. 



In the Allston area, structural discordances are many. Some of these 

 are possibly due to faulting. In a number of instances, however, the 

 melaphyr may be seen actually cutting the sediments. Dikes are known 

 in the field r^ear the bend in Commonwealth Avenue, in the woods to the 

 north, on Cambridge Street opposite Saunders Street, and in three places 

 in the conglomei'ate between Allston Street and Harvard Avenue. Two 

 of these latter dikes are, however, doubtfully regarded as melaphyr. 

 They have been mapped by Woodward as trap. 



Conclusions. — The facts set forth in this paper are believed to lead 

 to the following conclusions : 1. The melaphyr, in the region discussed, 

 is intrusive into the sediments. 2. The melaphyr is not associated with 

 any definite horizon, and is therefore of no value as a guide to the inter- 

 pretation of the structure. 



The first conclusion depends upon the following facts : 1. The con- 

 glomerate, associated with the melaphyr, contains no fragments of it. 

 2. The contacts, wherever found, are igneous in character. 3. The 

 melaphyr is seen in contact with sediments varying from the coarsest of 

 the conglomerate to the finest of the slate. 4. The distribution of the 

 melaphyr shows it to be discordant with the structure of the sediments 

 under any interpretation of the latter that has been offered. The second 

 conclusion follows directly upon the first. 



Incidentally, a new interpretation of the structure is offered for the 

 Chestnut Hill slate belt and the northern area of conglomerate. 



tol. xxxvm. — no. 2 2 



