180 ON MICA-TRAPS FROM KENDAL AND SEDBERGH. 
Discussion. 
The Presipent thought that very commonly the method suggested 
by the author for discriminating between calcite and dolomite under 
the microscope was not applicable to limestone rocks. 
Mr. Rurtzy said he was acquainted with the rocks described, and 
that he entirely agreed with the author in his conclusions. He 
pointed out that many of the Lake-District dykes mapped by the 
Geological Survey as felstones should be regarded as such, or as 
micaceous eurites. 
Prof. Sretry thanked Prof. Bonney for clearing up many of the 
difficulties connected with minette which he had himself in vain 
endeavoured to grapple with without the aid of the microscope. 
He offered the suggestion that some minettes may have been formed 
by the consolidation of masses of volcanic dust that had fallen and 
filled, or been compressed or washed into, fissures. He cited in- 
stances in the Eifel of masses of volcanic dust containing crystals 
of mica and other minerals in every stage of development, so as 
to present an external approximation to minette. He suggested 
that some of the specimens which were described by Prof. Bonney, 
containing, as they did, so large a proportion of carbonate of lime 
and other evidences of decomposition, might have so originated. 
Dr. Hicks asked if the dykes of minette altered the adjoining 
rocks. He suggested that they might result from the metamorphism 
of mica-slate, and have been forced from below into fissures. 
Mr. Rurtey confirmed the true eruptive character of the dykes 
and bosses of minette referred to in the paper. 
Dr. Suerener had lately studied the minettes of the Vosges, and 
was able to confirm Prof. Bonney’s views as to the true igneous 
origin of those rocks. In the Vosges similar dykes occur in the 
Precarboniferous greywacke and clay-slates of the Vosges, and the 
rocks on either side are perceptibly altered. 
Mr. W. W. Smyru stated that somewhat similar micaceous rocks 
had been described by Mr. Collins as occurring a few miles E. of 
Truro in Cornwall, and as being plutonic rocks constituting very 
flat-lying dykes. For those rocks Mr. Collins had, perhaps un- 
fortunately, proposed a number of new local names. 
Prof. Bonney replied to the President that, although the characters 
described in dolomite were not always found, yet when found they 
distinguished it from calcite. His real difficulty in dealing with 
these rocks was owing to the indistinct crystallization or de- 
struction of the felspars. He did not know whether he ought 
to treat Prof. Seeley’s suggestion as serious, especially as he had 
not examined the volcanic dust with the microscope. He said that 
the dykes in question sometimes narrowed from below upwards, 
and none of them ever contained any trace in microscopic sections 
of the clearly marked features of volcanic dust, which were seldom 
obliterated in altered rocks. He replied in the same way to Dr. 
Hicks’s suggestion, and stated that there were not the smallest 
grounds tor regarding these as otherwise than truly igneous rocks. 
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