292 C. H. Smyth, Jr. — Dihes of Alnoite at Manheim^ N. T. 



strongly pleochroic and in basal sections often has a marked 

 zonal structure. In spite of the fresh look of the rock in hand 

 specimens (which is due to the great abundance of mica), not a 

 grain of unchanged olivine has been found, and the determi- 

 nation of the mineral is based wholly upon the form and com- 

 position of the alteration product, and the undoubted presence 

 of olivine in the neighboring dikes. It is now represented by 

 serpentine of deep green color. Less often a phenocryst is 

 clear and colorless, presenting at first glance the appearance 

 of unchanged olivine, but, with crossed nicols, showing aggre- 

 gate polarization in low tints. 



In- addition to the two chief constituents, magnetite and apa- 

 tite are present in some quantity, while perofskite is less 

 abundant. Large amounts of secondary calcite are shown in 

 nearly every section. 



The most striking feature of the rock is the scarcity or 

 absence of melilite. In the other two dikes this mineral is 

 abundant, though often greatly altered ; but in the large dike 

 it has not been positively identified, although its presence is 

 occasionally indicated by obscure alteration products. 



This peculiar fact admits of two explanations. It is possible 

 that, owing to the slow crystallization of the material of the 

 large dike, or to some other variation of conditions, melilite 

 was never formed, its elements entering into other combina- 

 tiotis. If this were so, hornblende or pyroxene might be 

 expected in the rock, and none is shown. It is possible, how- 

 ever, that such a mineral is now represented by a part of the 

 serpentinous alteration products. 



The second explanation of the absence of melilite ascribes it 

 to the complete removal of the mineral by the processes of 

 alteration. This supposition is in harmony with the well- 

 known susceptibility of the mineral to alteration, which is well 

 shown in the other dikes, where much of the melilite is so 

 altered as to entirely obscure its original properties. Neverthe- 

 less, even here, much clearer traces of its form and structure may 

 be seen than in the large dike, where they are usually absent. 



Between these two explanations it is impossible to decide 

 with absolute certainty. But in view of the close proximity 

 of the three dikes, the similarity of conditions that must have 

 prevailed during their consolidation, and the identity of all the 

 other minerals, together with the excessive alteration of the 

 large dike, it seems extremely probable that melilite was origi- 

 nally present in the latter. On this ground it is included with 

 the other two dikes under the term alnoite. 



That still other dikes of a similar character exist in the 

 vicinity can hardly be doubted, but their extreme susceptibility 

 to weathering combines with the extensive covering of glacial 

 material to conceal them from observation. 



■Hamilton College, Clinton, N. Y., June, 1896. 



