268 GEORGE C MATSON 



Neither section shows more than one or two crystals of pyroxene, 

 and careful study does not indicate that any large amount was ever 

 present. For purposes of comparison, the calcite and serpentine 

 may be considered as olivine, for it is apparent, from the form of the 

 grains, that they occupy the position of the original olivine grains. 

 The sum of the serpentine calcite and olivine is, for No. I, 71.713 per 

 cent.; for No. II, 56.978 per cent. If these represent the percentages 

 of olivine in the original rock, the evidence of magmatic differentia- 

 tion is striking. A similar result is obtained by a comparison of the 

 percentages of biotite in the two sections. The comparison of the 

 magnetite and ilmenite is not a safe means of judging, because No. I 

 shows more weathering than No. II, and much of its magnetite has 

 been removed by solution. 



The composition of the other dikes shows considerable range, but 

 all may be classified under the head of mica-peridotite — a name 

 proposed by J. S. Diller 1 for a peridotite of similar composition 

 occurring in Crittenden County, Ky. 



Contact phenomena. — Along the contact the shaly sandstone and 

 shales are slightly hardened. Near the Cascadilla and Glenwood 

 dikes the rocks are traversed by an unusually large number of joints. 

 There are many irregular cracks in the sedimentary rocks near the 

 contact of most of the dikes. These cracks are filled with calcite. 

 No new minerals occur in the shales, though near the contact they are 

 abundantly impregnated with calcite and are stained with limonite. 

 The contact is irregular, and frequently the lava has extended in 

 long, thin tongues into the bedding planes. This shows the great 

 fluidity of the magma. The same thing is shown in the small dikes 

 at Taghanic where they cut the Genesee shale. There they fre- 

 quently pinch and swell very rapidly, and often branch from one 

 joint plane to another where two joints intersect. Frequently 

 branches of a dike pass on both sides of a fragment of shale which 

 is thicker than the combined width of the two branches of the dike. 

 The great fluidity of the lava in such cases does not necessarily mean 

 an excessively high temperature, as basic lavas liquefy at compara- 

 tively low temperatures. The meager contact effect is explained by 

 thinness of dikes and their low fusion point. 



1 American Journal of Science, 3d Ser., Vol. XLIV (1890), pp. 286-89. 



