308 Analyses of Books. [Oct. 



If beds were originally horizontal, and afterwards shifted, the 

 supposible causes of this shift are : 1. An internal force acting 

 frorn below upwards so as to raise the crust of the globe. 2. A 

 want of support, owing to internal cavities, so that the beds have 

 fallen by their own gravity. 3. An external shock, which has 

 broken the shell, and made one part tumble over another. 

 Dolomieu was inclined to the last of these opinions ; but if the 

 inclination be original, to what circumstances can we ascribe it ? 

 .The following are the explanations that have been attempted : 



1 . Hutchinson ascribes the inclination of slate to the shape of 

 the component particles. 



2. Manv authors ascribe the inclination of the strata to the 

 inequality of the base on which they were deposited. 



3. Werner considered the primary rocks to be crystalline and 

 stratified ; the transition rocks to be partly crystalline, partly 

 mechanical ; and the secondary as mechanical. 



There are many beds of rocks which are subject to curvature 

 . and angularity. Gneiss, mica slate, chlorite slate, clay slate, old 

 . red sandstone, calp, limestone, and alluvial sand, may be men- 

 tioned as well-known examples. 



These curves are sometimes exceedingly large, sometimes 

 exceedingly small ; and very often they constitute curves of 

 double curvature. These curves have been ascribed to the 

 following causes : 



1. To the strata being lifted while flexible and ductile. But 

 this explanation cannot be admitted, because curved strata often 

 lie upon horizontal strata. 



2. To crystallization. But the curvature of contiguous strata 

 composed of different materials seems fatal to this opinion. 



Our author thinks that the curvature of strata is owing to 

 three different causes. 1. To the unequal effects produced by 

 temperature on the materials. 2. To the motion of the fluid 

 from which they were deposited. 3. To the shape of the ground 

 on which they were deposited. 



Mr. Greenough terminates this essay as follows : 



" To conclude, then, let me ask, where a rock is stratified ; 

 is it necessarily bounded by parallel surfaces ? If so, let us hear 

 no more of mantle-shaped, saddle-shaped, shield-shaped, fan- 

 shaped, basin-shaped, trough-shaped stratification. 



"Are its surfaces necessarily parallel to those of the adjoining- 

 rock? If so, let us hear no more of unconformable and over- 

 lying stratification. 



" Is it sufficient that parallelism shall be found in a portion of 

 the rock ? Let us never hear of substances being unstratified. 

 Or must it extend throus;h the entire mass? Let us hear no more 

 of strata. 



" The laminse of flagstone, the folia of slate, are these strata ? 

 Are laminas, 400 yards thick, strata? Is there any assignable 

 limit to their thickness or tenuity ? 



