GEOLOGY. 227 



Silurian schists of Scotland, Ireland, Portngfal, and also in Xew York 

 State. But Inostranzeflf has shown that a large bed of anthracite, 

 which is worked on a large scale in the Saoneshje region of Eussia, be- 

 longs to the Huron age. He finds four kinds of carbonaceous mate 

 rial, and among these graphite is found. This carries the coal period 

 back into the archcean, and is interesting as throwing some light on the 

 formation of the graphitic schists. There is, then, no break in the line 

 of coal-bearing rocks between the crystalline schists, and the rocks 

 bearing tertiary lignites or the recent peat beds. 



The Pennsylvania geologists do not think that position, with reference 

 to the disturbing forces that have elevated the Appalachian Mountains, 

 will alone account for the formation of anthracite. The coal deposits 

 do indeed become more bituminous to the west, but not in such prox)or- 

 tion as would be expected if disturbance were the cause. The character 

 of the strata which cover them, and the depth of burial, are of much 

 importance in determining the present nature of coal beds. 



CLAY SLATE. 



The origin of slaty cleavage, except so far as that it results from press- 

 ure has never been settled. Sorby mixed micaceous hematite with clay, 

 and by pressure he caused these scales all to assume a common posi- 

 tion, and he argued that such pressures, acting in given dn^ectious, could 

 give the structure to any of the schists, and cleavage to the slates by 

 simply arranging elongated or flattened particles. Tbis experiment has 

 often been repeated with various modifications, and it surely is a fact 

 that such i)ressures acting as indicated have produced many features 

 that we see in the rocks. Tyndall, however, considered slaty cleavage 

 a result of the flattening of particles. Mr. E. W. Hilgard, with agri- 

 cultural ends in view, has made experiments upon the flocculation of 

 particles, and he shows that fine sediments suspended in agitated water 

 tend to flocculate, and form little masses which, acted upon by compres- 

 sion, are easily flattened. He, therefore, is strongly of the opinion 

 that Professor Tyndall is correct. It may farther be said concern- 

 ing clay slates, that microscopic study lias tended to modify exist- 

 ing opinions concerning these. They have been usually considered as 

 hardened indurated clays, and classed among fragmeutal rocks. The 

 microscope reveals in these rocks, however, crystalline elements which 

 Kalkowski has thought he proved to be staurolite, and which have been 

 thought by van Werveke and others to be of rutile. The fragmental 

 nature of the rock is very difficult to establish, and with these evidences 

 of crystalline structure there is no reason to think that most slate is 

 anything more than an excessively fine grained kind of mica schist, 

 made perhaps by the same forces acting in a less efficient manner. 



CRYSTALLINE SCHISTS. 



I think it can be said that but little progress has been made during 

 the past two years in the study of the crystalline schists, so far as the 



