ON PERCIVAL'S GEOLOGY OF CONNECTICUT 



Rock near New Haven, and Mount Tom in Massachu- 

 setts, were parts of one continuous trap range. His ob- 

 servations early showed that this was wholly an error; 

 that there was no one line ; that, on the contrary, many 

 ranges existed having the same general north and south 

 course ; and, moreover, that each was made up of a series 

 of isolated parts. These trap rocks of Connecticut, as 

 has been well proved, and as was early indicated by 

 Professor Silliman, are intrusive or igneous rocks, rocks 

 that fill fractures of the earth's crust, having come up in 

 a melted state from the earth's interior at the time when 

 the fractures were made ; and hence Percival's observa- 

 tions proved that there had been, not one long-continuous 

 fracture through the State from New Haven to the regions 

 of Mount Tom and beyond for the ejections of liquid 

 trap rock, but, instead, a series of openings along a com- 

 mon line, and that there were several such lines running 

 a nearly parallel course over a broad region of country. 

 He also found that the ridges which compose a range do 

 not always lie directly in the same line, but that often the 

 parts which follow one another are successively to the 

 east of one another, or to the west (en e'chelon, as the French 

 style it); and further, that the parts of the component 

 ridges of a range were often curved or a succession of 

 curving lines. He discovered, too, that in the region of 

 the Meriden Hanging Hills the trap ridges take a singu- 

 lar east and west bend across the great central valley 

 of the State, a course wholly at variance with the old 

 notions. 



' The work which he accomplished was, in the first 

 place, an extended topographical survey of his portion of 

 the State ; and, secondly, a thorough examination of the 

 structure and relations of the trap ridges, with also those 

 of the associated sandstone. And it brought out, as its 

 grand result, a system of general truths with regard to 

 the fractures of the earth's crust which, as geologists are 

 beginning to see, are the very same that are fundamental 

 in the constitution of mountain chains. For this com- 

 bination of many approximately parallel lines of ranges 

 in one system, the composite structure of the several 

 ranges, and the en echelon, or advancing and retreating 

 arrangement of the successive ridges of a range, are com- 

 mon features of mountain chains. The earth's great 



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