IOWA ACADEMY OF SC1P:NCP:S. 35 



The speci6c gravity of fresh looking, well formed crystals was found to be 

 0.6002. A chemical analysis kindly furnished by the Missouri State Geological 

 Survey, made by the St. Louis Sampling & Testing Works, gave the following 

 results, which leaves no doubt but that the mineral in question is a lime-iron 

 garnet, of the variety melanite. although it is more of a brown in color than that 

 variety usually is: 



100.30 per cent. 

 The occurence of this mineral is of interest because it seems to be the first time 

 it has been found in the State, and because garnets of all kinds are so rare. In 

 fact, with the exception of one instance reported to the writer in a private com- 

 munication, by Prof. (t. C. Broadhead, and which has not yet been published, this 

 is the only locality known in the State of Missouri where garnets of any kind are 

 found. In Bulletin No. 5, of the Geological Survey of Missouri, p. 42, it is stated 

 that garnets have not been found anywhere within the area of the crystalline rocks 

 of the State. It should be noted that, occuring as they do here within a dyke rock, 

 they in no way have a bearing on the question there discussed, viz., the origin 

 of the granites and porphyries. 



II 



At diiTerent places in Southeast Missouri some of the many fine specimens of 

 calcite are coated with a thm film which is a beautiful, rich amber in color. An 

 examination of this coating showed it to be a compound of ferric oxide the exact 

 •chemical nature of which was not determined. One of the specimens from Potosi, 

 in Washington county, which has been in the Penn College Museum for a few 

 years, so well illustrates, not only the controlling force a crystal has over 

 the molecules of a pseudomorph deposited on it, but also the law of symmetry 

 in crystallization, that it is thought worthy of mention. It is a collection of modi- 

 fied left handed scalenohedrons from one to two centimetres, for a half length, for 

 the most of which the formula — 2 R~ was estimated. Usually each scalenohedron 

 is terminated by rhombohedral faces. In addition to this each acute polar edge is 

 beveled by a second scalenohedron producing another set of six faces from one to 

 three millimetres wide. The coating on these 

 crystals is comparatively light and the mole- 

 cular control exerted by the calcite has caused 

 it to be deposited on these narrow-scale no- 

 hedral faces only, leaving the remainder of 

 the crystal entirely unaffected. Fig. 5 illus- 

 F'S. ii- trates this. 



By comparing this specimen with others it is found that as the coating became 

 thicker it was next deposited on the rhombohedral faces, and covered the whole 

 crystal only after it became so abundant that the molecular force of the calcite 

 could no longer control it. This is the finest (xample the writer has ever seen of 

 a crystal controlling a pseudomorph deposited on it, and especially illustrating so 

 well at the same time the law of symmetry in crystallization. 



