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granites and felsites, as well as some of tlic nielapliyrs and por- 

 phyritcs ; and since some of* these rocks, and especially the 

 granite, are clearly eruptive at many points through the Lower 

 and Middle Cambrian slates of the Boston Basin, it follows 

 that the Ilinghain slate should be referred to a higher horizon. 

 I have also observed in this conglomerate frequent pebbles of 

 slate, a further confirmation of the view that there is an older 

 slate in this region. Of especial interest in this connection, 

 however, are the calcareous portions of the conglomerate. 

 The most casual observer would l)e likely to notice that limited 

 portions of the coarse conglomerate have an etched ap[)earance, 

 weathering in the smooth and cavernous manner peculiar to 

 calcareous rocks, and the test witli acid at these points shows 

 an abundant calcareous cement, the solution of which allows 

 the pebbles to fall apart. This calcareous conglomerate is 

 chiefly in the form of irregular but rounded and sharply defined 

 masses or patches from a foot to a yard or so across : and else- 

 where in the bed the calcareous cement is wholly wanting. This 

 is analogous to what may sometimes be observed in the modified 

 drift of limestone districts, such as western New England, beds 

 of loose sand and gravel enclosing irregularly rounded but often 

 extremely graceful masses of firm sandstone and conglomerate 

 due to the solution and segregation of limestone debris. 

 The source of the calcareous material in the Hingham conglom- 

 erate is an interesting question. Does it indicate contempora- 

 neous shells and corals — fossils belonging to the same geological 

 a"-e as the consflomerate, or is it, as in the more modern exam- 

 pies just cited, limestone debris from some older formation, 

 wliich, after its enclosure in the conglomerate, has undergone 

 segregation — solution and deposition — so as to assume a concen- 

 trated form in limited portions of the rock? The latter is 

 undoubtedly tlie true view ; for there still remain undissolved 

 fragments of the limestone, well-rounded distinct pebbles of all 

 sizes up to a foot or more in diameter, some of them beautifully 

 stratified in directions entirely at variance with the bedding of 



