130 W. M. Hutchhigs — An Interesting Contact- Rock. 



given bed affected is not sufficient to allow of comparisons. Thus, 

 the bed we have specially considered is eight feet thick. Its 

 alteration is as intense at the bottom as it is at the top, and chemical 

 tests also show that no differences occur on which stress could be laid 

 in this connection. And so with the other beds affected. But we 

 know that clays and shales in the Carboniferous beds invariably 

 contain much more potash than soda, say from three to four times as 

 much. This is fully borne out also among the rocks at the Whin 

 Sill. Some of the shales of which I have made analyses show that 

 no chemical change has taken place in them, and in such cases the 

 •potash and soda ax'e in the normal proportions, — as near as may be, 

 for instance, as in the analyses published by me of clays from thei 

 Ooal-measures. 



When in these beds we find altered shales or clays with more 

 soda than potash, or even with an approach to equality in these 

 bases, we are quite safe in concluding that this striking alteration 

 in the normal proportions is due to the intruded soda-bearing rock 

 in some way. This is the case with the rock we are now con- 

 sidering; the soda is considerably in excess of the potash. As to how 

 the transfer has taken place in these cases, there seem to be three 

 possible causes to be considered. It may have taken place at the 

 time of the intrusion, by actual passage of igneous magma; it may 

 have been effected after intrusion, and during the subsequent heating 

 and cooling of the sedimentary rocks, by the passage of hot aqueous 

 or vaporous compounds, or both ; or, it may be brought about after 

 complete cooling, and long after all " contact-action," by the perco- 

 lation of water from the igneous rock during weathering. 



The first of these conditions may apply, to a limited extent, close 

 to contact. It certainly does not apply to the bed now in question, 

 which is 75 feet from contact, and separated from it by several other 

 filternating beds of shale, limestone, and sandstone. The third 

 condition may frequently apply to a small extent and sometimes to 

 a considerable one. But, in the present special instance (and in 

 others in the same district), microscopic study of the rock precludes 

 the supposition that subsequent percolation has deposited soda- 

 compounds in it. When we have a specimen consisting of a hard, 

 compact, felsite-like mass, which the microscope shows to consist 

 of a mosaic of quartz and felspar, we cannot readily understand 

 how the original rock could be removed, and this deposited, by 

 percolation. And still less can we do so when we have a rock 

 consisting of quartz, felspar, isotropic matter and white mica. It 

 would be different if it were more or less cracked and rotten, and 

 showed zeolites in it, but of this there is no trace. 



We must deal with the fact that transfer does take place, in spite 

 of the many difficulties in the way of understanding exactly how it 

 is effected ; such difficulties as arise when we have to note that in 

 one given section of beds there will be cases in which evidence of 

 transfer is wanting, though beds further from the contact are strong 

 in such evidence. Such difficulties are, at any rate, not greater 

 than, and are indeed analogous to, others which we have to face 



