26 IOWA ACADEMY OF SCIENCES. 



it is only the outside which is changed, to the depth of from two to four centi- 

 meters, or more, the interior still often preserving the rock in its original character, 

 so that no doubt arises concerning its composition and structure previous to its 

 embedding in the granite. The contact zones are in all respects identical with 

 the contact belts of other localities where acid eruptives have pushed up against 

 the same kind of rocks. 



Chemical analyses of the unaltered inclusions, the metamorphosed shells and 

 the surrounding granites show that the altered shells have an acidity intermediate 

 between the inclusions and the granites. 



These proofs of eruptive origin of the Maryland granites are quite similar to 

 those -which Barrois* has formulated from granites of Rostrenen. 



(4) Microscopical Examinations.— ks'ide from the ordinary microscopical char- 

 acters indicative of cooling from fusion, certain of the granites under considera- 

 tion show some additional phenomena pointing to the same end. These are large 

 grains of micropegmatitic intergrowths of quartz and feldspar rounded through 

 magmatic corrosion apparently and having the characteristic embayments so 

 commonly associated with Cases of this kind. 



STRATA BETWEEN FORD AND WINTERSET, 



BY J. I>. TILTON, INDIANOLA. 



[The following article was accompanied by a series of diagrams representing the 

 size, location, and relative position of the various out-crops.] 



Middle river rises on the eastern slopes of the divide in Adair and Guthrie coun 

 ties. It flows just south of Winterset, in Madison county, then northeasterly to 

 the northeast corner of Warren county, where it takes an easterly direction for four 

 miles and flows into the Des Moines river, about eight miles below the city of Des 

 Moines. Consequently, a line drawn along Middle river from its mouth to Winter- 

 set, a distance of about fifty miles, passes from close to the lower strata of what 

 White calls the "Middle Coal Measures," across the entire series of both the 

 " Middle" and " Upper Coal Measures." In the sections iound along this line we 

 may ascertain the local thickness of the different strata, some facts in regard to the 

 continuity of the different strata and of the different seams of coal, also the posi- 

 tion of the border between the " Middle" and " Upper Coal Measures;" or between 

 the "Lower" and "Upper Coal Measures," following the classification that will 

 probably be accepted. 



In the diagram! before you the different out-crops are so drawn by a scale as to 

 represent the relative'thickness of each of the strata, their distances apart and loca- 

 tion. These diagrams are so placed side by side as to represent the continuation of 

 the strata. 



The explanation accompanying each stratum describes the surface appearance at 

 the out-crop, regardless of what the texture of the stratum may be where atmos- 

 pheric agencies have had less chance to work than at the exposure; yet, comparing 

 the out-crops of the same stratum in sections adjacent to each other, we see in various 

 places a change in structure not to be wholly explained by the action of atnios- 

 . *Ann. de la Soc. geol. du.Nord, t. XII, p. 106. 1885. 



