COMPARATIVf: INITIAL TEMPERATURES 377 



appearance in the succeeding paper, and we find there that 2/./?/.^ was 

 .5055, K is about 3, and k/cn Uo is .00002. 



Suggestions as to comparative Initial Temperatures 



It is worth noting in comparing these rocks, which are in some respects 

 so similar, that the initial temperature is nearest the temperature of the 

 consolidation of the augite in the overflow. It is higher in the intrusive 

 Palisade trap, still higher in the small dike, and highest of all in the 

 large dike, which realW might almost be classed as a gabbro. It is easy to 

 see that if the initial temperature were a little higher we should have no 

 marginal zone of finer grain whatever. This would be true if the molten 

 magma were initially hotter or if the country rock were hotter, so that 

 it would probably be true of an}" rock injected at a greater depth than 

 the Medford diabase. 



It may be worth while to remark also that if Lord Kelvin's theory 

 that the earth is a cooling globe is correct, then the depth at which plu- 

 tonic conditions of grain would be attained would have been much less 

 in earlier geological times than at the ^^resent day ; for at a given depth 

 the country rock would have been hotter and the magma from a given 

 depth below would also have been hotter. This may be another reason 

 beside their greater exposure b}" erosion why plutonic rocks are rather 

 more frequent in the earlier ages. 



Other Minerals of the Marquette Dike 



Returning once more to the Marquette dike, we will describe what we 

 see in the sections without burdening our text with any more figures and 

 computations. 



Taking the feldspar first, we find that sections within 2 or 3 inches of 

 the margin show a pronounced porphyritic texture. Many of the crys- 

 tals have obviously been formed while the magma was still in motion, 

 and are arranged most abundantly with their flat sides more or less 

 parallel to the margin. At the same time there is really no absolute line 

 to be drawn between these phenocr3^sts or rhyocrj^stals and the feldspar 

 of the groundmass. This is especially true as we go farther from the 

 margin. Even at 100 or 200 millimeters from the margin there is still 

 some trace of the rhyocrystals, but by this time there is no definable 

 distinction in size, and by the time we have got 600 or 700 millimeters 

 in two generations are no longer definable. The natural inference is 

 that the feldspar for a few inches next the wall began to crystallize before 

 the magma had come to rest. As we continue our study of the grain 

 toward the center, we find the feldspar becoming coarser and coarser, 



LIII— Bum,. Geot.. Soc. Am., Vol. 14. 1902 



