236 MINERALOGY AND LITHOLOGY. 



In view of the fact that rocks of this class have been shown to be 

 widely distributed over the earth, forming the strata of the Huronian 

 period, Dr. Hunt suggests it as desirable that those who investigate these 

 rocks should study them with the view of finding whether certain char- 

 acters cannot be found to exist in them, which are so general as to serve 

 as an equivalent to a determination of their age. I wish to recognize fully 

 the value of such considerations, and the interest attached to the problem 

 of the kind of changes that may have taken place in sediments at differ- 

 ent periods of the earth's history ; but it will be noticed that the result 

 of these studies has been, to lead to the conclusion that the nature of 

 these rocks is simply dependent on the geographical location of the Con- 

 necticut valley with reference to the surrounding land, which determined 

 the nature and condition of the original sediments. The composition 

 being thus fixed, their present condition is dependent on their location 

 with reference to the action of the forces producing metamorphism. As 

 sediments of the same nature are accumulating to-day, it therefore appears 

 plain that location with reference to surroundings might produce the same 

 kind of rocks in any era which is characterized by crystalline schists; 

 and therefore the problem of the determination of age is thrown entirely 

 upon geologists in the field, for in this case lithology can point to no rea- 

 son for the maintenance of uniform conditions in different localities. To 

 me. Dr. Hunt's studies on the condition of sediments, as regards their 

 permeability by water, is a more important consideration in the study of 

 these rocks, for the fineness and argillitic nature of many of the strata 

 are indications of impermeable sediments which would protect the solu- 

 ble materials in included basic layers, and preserve in them the condi- 

 tions that are necessary for the formation of the basic rocks which are 

 now found among the strata. 



CLAY SLATE (ARGILLITE). 



Between argillitic mica schist and clay slate or argillite, no dividing 

 line exists. Typical clay slate is a fragmental rock, which consists of 

 the consolidated, nearly dehydrated material of clay beds; but the ap- 

 plication of microscopic study to these slates has indicated that they do 

 contain certain crystallized elements, which though very minute show 



