MARYLAND GEOLOGICAL SURVEY 139 



The analysis of this rock bears a marked resemblance to that of 

 the quartz-monzonite, into which it intrudes. This analysis has been 

 tabulated with it for the purpose of easy comparison. 



The difference lies in the greater acidity of the dike rock, the 

 silica percentage is higher and the lime percentage is lower than in 

 the monzonite. In both rocks the soda percentage is greater than 

 the potassa. This difference is more marked in the secondary intru- 

 sive. 



The chemical difference is just such as would be expected between 

 a large intrusive body, representing an early stage in magmatic differ- 

 entiation, and the material which fills fissures in this body and which 

 represents a subsequent stage of differentiation. 



The feldspar was calculated as Ab 12 An x . The extinctions that 

 were secured were high on 010 (15°-20°), and very low on 001 

 (3°-4°). 



The lime that remained after the calculation of the anorthite mole- 

 cule and apatite and titanite, was referred to the garnet. This con- 

 stituent is distributed in microscopic crystals, somewhat sparsely 

 throughout the slide, and can scarcely have developed at the expense 

 of the feldspar. It is considered a primary constituent. It does not 

 occur where the rock has been altered to a greenstone. 



The chemical composition, the mineral constitution, and the struc- 

 ture of the rock is that of a rhyolite. Its alteration is so extended 

 and has been of such a character as to obscure or obliterate the 

 original structure, hence the rock must be classed with the meta- 

 rhyolites. 



MICRO-GRANITIC DIKES. 



Within the meta-rhyolite are dikes of varying width of a fine- 

 grained hornblende-granite containing quartz, albitic feldspar and 

 hornblende with accessory magnetite. 



A determination of the alkali, lime and silica shows the rock to be 

 of about the same acidity as the more basic quartz-monzonite of the 

 granitoid belt, but richer in soda. 



