Maryland Geological Survey 171 



ratio of sand and clay in the sample, a bed high in clay and low in sand 

 containing almost as large a proportion of coarse material in the sand as 

 does a distinctly sandy bed. With this wide range in the size of the sands 

 there probably also goes, generally, a high ratio of heavy to light minerals, 

 and to a certain extent an abundance of magnetite (c/. samples 1-3). 



This statement concerning the magnetite is made somewhat doubtfully 

 because there is definite evidence for it only in sample 1 ; it may be 

 true also of sample 2), but there the mica is so dominant as to leave 

 the percentage of minerals, more certainly classed as heavy by their 

 settling properties, relatively small, and it may also have caused the 

 magnetite to be overlooked. Abundance of magnetite is, moreover, char- 

 acteristic of sample 11, which must be regarded as a rather typical 

 example of the estuarine type. In the differentiation, at least of two such 

 closely related types, therefore, the proportion of magnetite must not be 

 given much weight. A high percentage of heavy minerals in general 

 seems more likely, however, to be a characteristic of the delta type. 



A great abundance of carbonaceous matter is another characteristic 

 of this type, for which, however, the evidence given in these analyses is 

 only qualitative. With this goes the formation of pyrite, or more prob- 

 ably marcasite, which, as will be shown later in the discussion of glau- 

 conite, is an alternative product to glauconite, formed in the presence of 

 abundant organic, especially humus matter. It should be noted, how- 

 ever, that as abundant humus matter is also characteristic of many 

 estuarine deposits, so marcasite is found also in these (samples 11 and 

 13). Furthermore, since the recognition of an opaque mineral of this 

 kind under the microscope is difficult, it is probable that it has been over- 

 looked in some samples in which it might be found if it were especially 

 sought. 



Finally, the form of occurrence in the field is very important for the 

 differentiation of this type, which is characterized by thin-bedding, by 

 extreme difference in the proportion of sand and clay in adjacent beds, and 

 by the occurrence of thin sand partings representing, doubtless, tempo- 

 rary stream floods. Moreover, in the argillaceous beds the abundance of 

 mica is usually a conspicuous feature in the field, little streams of carbon- 



