EASTERN ly^ASSACHUSETTS. 



487 



known as the Brighton amygdaloid. He held that a " trained eye " 

 could trace the gradual passage from the conglomerate to the mclaphyr, 

 remarking as follows : 



** We see .... a mass of conglomerate essentially the same as the Koxbury 

 pudding stone, only the pebbles and the cement have been greatly affected hy 

 heat, so that the whole is more fused together than in the ordinary forms of 

 that conglomerate — looking closely we see that the matrix of the pebbles 

 and to a certain extent the outer parts of the pebbles themselves are filled with 

 cavities in which similar amygdules have been formed. With care and with 

 favorable conditions of the quarries, the observer may trace the stages of this 

 transition, from the faintest beginning of tins' structure in rocks which are 

 distinctly conglomerates, into rocks where the blebs have been so completely 

 developed that every trace of the original pebbly structure is now lost, and 

 the mass converted into the amygdaloidal trap." (Proc. Bost. Soc. Nat Hist., 

 1879, XX., pp. 129-133.) 



From this he argued that the Roxbury conglomerate had been deeply 

 buried under sedimentary deposits, forming lofty mountains, which had 

 since been worn away to their foundations, the conglomerate in the 

 mean time undergoing fusion, and being thus converted into mclaphyr. 



Later, the question of the nature of the Brighton mclaphyr (amyg- 

 daloid) was taken up by Prof. E. 11. Benton, who showed that the 

 amygdaloid was a mclaphyr or an old altered basalt, which owed its 

 present variation from basalt to changes subsequent to its original for- 

 mation. The structure he regarded as pseudo-amygdaloid, and the 

 supposed pebbles and fragments were shown to be the same as the 

 remaining portion of the mclaphyr. He also points to the fact that Pro- 

 fessor Shaler's hypothesis requires the heterogeneous pebbles of the con- 

 glomerate to bo transformed into the homogeneous ones of the melaphyr ; 

 also, that no part of the mclaphyr is like the Roxbury conglomerate. 

 Professor Benton further showed that all the claims made in the past, 

 that the melaphyr passed into slates, conglomerates, etc., were without 

 any basis of fact, for a distinct line of separation between these rocks 

 could be found whenever they were seen together. (Proc. Bost. Soc. 

 Nut Hist., 1880, XX., pp. 41G-426.) 



Associated with the general belief in the production of crystalline and 

 eruptive rocks by the fusion of sediments is the theory of the plasticity 

 of pebbles in conglomerates, a theory at least as old as the days of 

 ]\tichael Kirwan. Certain forms observed in Brighton were regarded 

 by Mr. Crosby as the result of the compression of the quartzite pebbles 

 in a plastic state, but on examining the district in question Dr. Wads- 

 w^orth found that these forms were confined to glaciated surfaces, and 



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