1818.] ON TIIS FORMATION OF ROCKS. S05 



it is probable tliat they arc only the remains of an im- 

 mense bed of gypsum, which might at one time havft 

 occupied some part of the space where those passes and 

 valleys are formed. ' 



This gypsum has a small crystalline grain, with lit- 

 tle or none of the fibrous or lamellar crystallized gyp- 

 sum so common in the formations of the secondary 

 class. 



^7. Clay slate of transition. This shistoze forma- 

 tion, containing and alternating with strata which con- 

 tain impressions of vegetables, and, in some places, of 

 animals, must be considered as of Neptunian origin. 



A great variety of rocks, principally of a shistoze 

 structure, are included in this formation. They alter- 

 nate with shistoze limestone of transition, having small 

 veins of calcspar crossing the strata, the shist often 

 composed of small detached plates of mica, or what 

 has lately been called talc, and in some places small 

 veins of quartz intersecting the strata. It has the ex- 

 terior form of gneiss, Avhen the thin strata of blue cal- 

 careous shist, and plates of calcspar, in segments of un- 

 equal thickness, alternate with each other in the direc- 

 tion of the stratification. 



Roofing slate generally alternates with this forma- 

 tion ; and from its being best known, has contributed;^ 

 in a great measure, to include the others in the transi- 

 tion class. Being a necessary article for the covering of 

 houses, it has occasioned considerable quarries and ex- 

 cavations to be made in almost every country. In the 

 course of working and splitting the slate, impressions of 

 vegetables; fish^ &c. &c. were found; which probably 



