804 0. Harger-~New fresh-water Isopod. 



Perhaps I ought to have been more explicit in my statement, 

 but it seemed to me unnecessary, because on the assumption 

 of a solid earth the amount of elevation would be the same, or 

 nearly the same, whether, by the horizontal pressure, the strata 



chains) or only thickened without folding. If every two or 

 three parts in horizontal extent of sediments be crushed into 

 one part, there must be a corresponding thickening of the whole 

 squeezed mass, and, therefore, a corresponding elevation of the 

 surface, whether the strata be closely folded or only thickened 

 without folding. In reality, doubtless, both occur in every 

 case ; close folding in the upper parts and thickening without 

 folding in the deeper parts of the same squeezed mass. In fact 

 it is impossible that the folding should occur above without a 

 corresponding crushing and thickening below. 



Again, I am satistied that Prof. Dana greatly under-esti- 



mates the amount of elevation by simple mashing as c 



with folding: 1st, because folding is a superficial phenomenon 



and therefore always exposed to view, while crushing without 



folding is deep seated and only rarely exposed ; 2d, becausi 

 folding is ahvays revealed by stratification, while crushing is 

 only sometimes revealed by cleavage, for this structure is only 

 developed in suitable materials ; and 3d, because even after 

 folding, extension upward may take place by mashing to- 

 gether of the folds, as I have shown in the early part of this 

 paper. . 



I have spoken thus far of dosed folds. In open folds such as 

 occur on the skirts of mountain chains where the horizontal 

 crushing has not been sufficient to bring the folds together, the 

 ease might seem to be different ; but even in these there must 

 be a mashing of the strata below each anticlinal and propor- 

 tioned to its height, unless we assume a hollow arch beneath, 

 or else such an "arch supported by a liquid, an assumption 

 which is expressly set aside in my paper. 

 Berkeley, Oct. 11, 1875. 



Aet. XXXVIU.— Brief Contributions to Zoology from the Mu- 

 seum of Yah College. No. XXXYII.— Description of Mama- 

 sellus brachyurus, a new fresh water • Isopod ; by O. HaRGEB. 



The genus Asellopsis* was proposed by the writef for the 

 reception of Asellus tenax Smith, on account of the absence or 

 mandibular palpi. A second species of this interesting genus 

 has lately been collected by Mr. Fred. Mather, in Eockbridge 



