of Boston and its Vicimttf. 



»93 



II. ARGILUTE. 



I. The characters of Arslllitc have been already sufficlenllr 

 tletanctl^ page 177. It foims in Charlestown^ Watertown, Chel- 



sea and Quincy gcuily undulating eminences; but tbeir heigh L 



t\^ill not entitle them to tlie rank of hills. 



II. ArgilUte is stratified; the strata are horizontal. It is inter- 

 rupted by numerous parallel rents^ which have a two. fold dircc- 

 tion and oblkjuely intersect each other; hence ArgilUte appears 

 to be cut into rhomboidal tables. At the upper portion of the ele- 

 vations, the ArgilUte is often wholly composed of saiall regular 

 forms. The rents sometimes pursue various directions^ and di- 

 vide the ArgilUte into forms as various ; the sides of the rents are 

 sometimes separated a few inches from each other^ and the inter- 

 stice is filled with a kind of breccia, formed of angular fragments 

 of ArgiUite, cemented by ferruginous Clay. This aggregate forms 



sometimes floorings to veins of Lime. 



III. ArgilUte is the oldest rock which is to be observed, in 

 situ, in this vicinity. It is subordinate to Greenstone, in Charles- 

 lown, Brighton and Newton, and to Sienite in Milton and Brain- 

 tree. It passes into Novaculite, which forms an extensive bed 

 in it, at Charlestown, and into Petrosilex, at Dorchester and 

 Milton. 



IV. The hills where ArgilUte predominates are insulated, their 



bases being surrounded by an alluvion. 



y. Chlorite and Greenstone occur in ArgUlite in small beds ; 

 Calcareous Spar and Quartz traverse it in small veins ; some- 

 times an aggregate of Calcareous Spar and Quartz, with an Ar- 

 gillaceous basis, is found in small veins in ArgilUte ; the Quartz 

 is in small crystalline grains, and the Lime is intricately associated 



titJrhl 



