1882.] 131 [Wadsworth. 



them. Pebbles of this argillite were also found in the conglom- 

 erate, while the junction of the two is distinctly that of a conglom- 

 erate laid down on an older eroded surface. If the two were the 

 same formation it is difficult to conceive how the argillite and 

 conglomerate could be so absolutely distinct as they are. 



Naturally, if they were the same formation we should expect 

 to find a gradual passage between them, or at least a few pebbles 

 inclosed in the argillite. The supposition of a fault is not admis- 

 sible, because the two rocks were found united in a solid mass 

 with a definite line of contact. The unconform ability and the 

 occurrence of argillite pebbles in the conglomerate also forbid any 

 supposition except this : that the argillite was laid down, solidified 

 to its present condition, or nearly so, and then eroded; while 

 upon this eroded surface the conglomerate was deposited, nearly 

 coinciding in its strike and dip with the argillite. The " con- 

 glomerate" is found to be at its base, either a conglomerate or a 

 sandstone passing in places into an argillite. The denuded argil- 

 lite beneath is found both in pebbles in the conglomerate and in 

 material redeposited as argillaceous "mud, in and about the con- 

 glomerate, giving rise to bands and irregular patches of argillite. 1 

 Examination of quarries in the conglomerate in the vicinity of 

 Beacon and Station streets shows numerous pebbles of the under- 

 lying argillite in the conglomerate. 



Studies made by the writer at this locality and elsewhere in 

 the Boston basin, prove to him that there are at least two dis- 

 tinct argillites. One a fine grained and decidedly argillaceous 

 rock generally of gray color, like that underlying the conglom- 

 erate on Beacon street, and resembling in some of its characters 

 the Paradoxides argillite at Braintree. The other, of coarser 

 grain, often decidedly arenaceous, and generally of a gray, black 

 or reddish color. The latter argillite (oftentimes a true sand- 

 stone) is, so far as its relations have been ascertained, a component 

 part of the conglomerate (Koxbury). 



The slate rocks so well exposed in Somerville are of similar 

 character to this latter argillite, although their relations are not 

 definitely known. 



We have then at least two argillites in the Boston basin, to 

 the first of which the Paradoxides rock of Braintree most proba- 



1 Occas. Papers, Bost. Soc. Nat. Hist, in, 190. 



