1892.] Mineralogy and Petrography. 847 
MINERALOGY AND PETROGRAPHY:? 
Thermometamorphism.—A very brief but extremely interest- 
ing review of the effects of thermometamorphism in the acid and basic 
lavas in contact with granite and granophyre in the Lake District, 
Eng., is given by Harker.’ The altered zone surrounding the latter 
rocks is about ł of a mile wide. On its outer periphery only the sec- 
ondary constituents of the basic lavas have been affected by the con- 
tact action. This leads the author to the statement that the substances 
most susceptible to thermal agency are those formed under ordinary 
meteoric conditions, minerals of direct igneous origin being more 
refractory. In the rocks under discussion but little change in chemi- 
cal composition has taken place asa consequence of their metamor-. 
phism, except that there is a loss of water and a gain in boron quite 
near the contact. The mineralogical changes noted are the production 
of biotite from the chloritic decomposition products of pyroxene and 
the formation of clear feldspar from the original turbid mineral. In 
addition to these, quartz, green hornblende, actinolite, tremolite, augite, 
sphene, rutile, magnetite and pyrite have also resulted from the meta- 
morphic processes. The characteristic contact minerals of sedimentary 
rocks, cyanite, andalusite and garnet are practically absent. Their 
abundant presence in sedimentary rocks is thought to be due to the 
` fact that these contain but a small proportion of alkalies, as a result 
of the long continued chemical degradation to which they have been 
subjected, and that since feldspar, which the author regards as a char- 
acteristic contact mineral, could not form, only aluminous new 
products are possible. The careful study of the altered rocks indicates 
that there was very little interchange of substance between different 
portions of the original rock, except between those parts that were 
` immediately adjacent. The preservation of minute structures, such 
as fluxion lines and spherulitic aggregates, point to this conclusion. 
In the case of the acid lavas the material very near the metamorphos- 
ing mass consists principally of a fine-grained aggregate of feldspar 
‘and quartz. Aluminous and ferruginous compounds were absent from 
the original lava; they are likewise absent from its altered phases. 
'Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
Bull. Geol. Soc. Amer., Vol. iii, 1892, p. 16. 
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