284 



BULLETIN OF THE 



I believe that in Connecticut and elsewhere there are repeated outcrops 

 of the same beds, produced by faults as described below ; but their iden- 

 tification is now a difficult matter. Microscopic analysis and detailed 

 Btudy by local ob.'^ervcrs will do much to solve this diificulty. 



Composition of the Trap. — The composition of the eruptive rocks In 

 the Triassio belts is not discussed here, as the present work relates to 

 their physical characters. For this reason, the general term trap is em- 

 ployed throughout. Many names have been previously used, — whin, 

 greenstone, trap, basalt, sienitio basalt, diorite, trachyte, dolerite, and 

 diabase ; but the last two would seem the more proper ones, judging by 

 mineralogical composition. Chemical analyses and microscopic exami- 

 nations of the trap from various localities have been made by Cook, 

 E, S. Dana, Frazer, Hawes, Schweitzer, and Wurtz. The following 

 statement gives the limiting percentages of their results : — 



Silica 45.8-53.4% 



20.4% 



21.2% 



Alumina 12.5 



Iron oxides . 



7.8- 



Magnesia, Lime, Soda, Potash, less than 10% each. 



The minerals present are pyi'oxene, labradorite, and magnetite, with 

 certain accessory species, and chlorite as a common product of alteration. 

 E. S. Dana (391) and Hawes (a, 185) note an increase in hydratation 

 and alteration in going eastward across the Connecticut. It is notice- 

 able that the least modified traps are dikes or intruded sheets, and the 

 most modified are overflows, according to the present determinations ; 

 thus Hawes describes East and West Rocks and the Jersey City traps 

 as dolerites, and the Saltonstall and Durham Mountains (Conn.) as 

 diabases : Mount Holyoke gives an exception to this apparent rule, as 

 it is classed under the dolerites, although it is certainly an overflow. 

 The two authors above named considered all the trap as intrusive, and 

 consequently did not perceive the natural relationship between condi- 

 tions of origin and composition, as here suggested. 



Relations of the Trap and Sandstone. — Two views as to the ori^.n )f 

 the trap sheets have been discussed. According to one, they are con- 

 sidered eruptive across or between the sandstone layers, and more or 

 less active in aiding the breaking, tilting, consolidating, and coloring of 

 the strata, after the period of deposition had ceased. The other view 

 looks on the trap sheets as younger than the sandstones below, and 

 older than those above them; or, as it maybe stated, the sandstones 

 and traps are geologically contemporaneous ; the sheets are old lava 

 overflows buried under strata of later date than their eruption. 



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