'I 



? ! 



MUSEUM OF COMrARATIVE ZOOLOGY. 



Ill 



these facts, wlilch were first pointed out by Messrs. Foster and Whit- 

 ney, and later by Mr. Marvine, is that given by them, that the traps are 

 lava flows, and that they were successively laid down upon one another, 

 or covered by sandstone and conglomerate. They are seen to have the 

 same characters, except so far as they have undergone secondary changes, 

 that modern basaltic lavas have. These old basalts have been denomi- 

 nated mclaphyrs and diabases by Prof. Pumpelly, to whom lithologists 

 are indebted for their knowledge of their microscopic characters. While 

 we would use the terms that Prof. Pumpelly has, we object to the ap- 

 plication he has made of them. Many of his diabases we should call 

 mclaphyrs, and many of his melaphyrs wo should class as diabases. 

 We of course differ in our dofmitions of these terms, for wliile he would 

 regard melaphyr and diabase as distinct rock species, we hold that they 

 are only altered forms of basalt. The greenstone ridge back of the 

 Cliff and Phoenix mines we regard as an excellent example of diabase 

 (791, 792), with whicli we class all the hcavy-boddcd crystalline traps 

 of that region, while the thin-bedded scoriaccous or amygdaloidal highly 

 altered traps wo class as melaphyr, but in the majority of cases Prof. 

 Pumpelly regards them as the reverse. The diabases are rarely if ever 

 mined, the mclaphyrs frequently. 



In a recent paper "On the Carboniferous Volcanic Rocks of the 

 Basin of the Firth of Forth — their Structure in the l.eld and under 

 the Microscope,"* Prof. Geikie points out the difference microscopically 

 between lava flows and intrusive masses, and evidently thinks that 

 they can always be as readily separated in the cabinet, microscopically, 

 as they can be' in the field. While it is doubtless true that this sepa- 

 ration can be made in the rocks, studied by Prof. Geikie, his distinc- 

 tions fail in the Lake Superior district. The difl'erence in structure 

 pointed out by him seems to be entirely owing to the rate 'of motion 

 and pressure at the time of crystallization, and the rapidity with Avhich 

 the lava solidified. When lava flows in thin sheets, or, if we con- 

 fine our examination to the upper portion of a thick sheet, we find 

 characters that readily distinguish the sheets from the dikes ; but when 

 we come to study the middle and lower portions of the thick sheets, 

 where there was little or no motion combined with the pressure of the 

 overlying mass, the rock is undistinguishable from rock of the same 

 composition in dikes ; and tlie diagnostic characters given by Prof 

 Geikie would assign it to an intrusive rock,: not to an overflow. In the 

 before-mentioned Emerson adit at the Copper Falls mine, some mela- 



# ' 



Trans. Koyal Soc. of Ediiilmrgli, XXIX. 437-518. 



