Correspondence—Prof. T. G. Bonney. 285 
When a large mass of molten matter occurred near the surface, 
and a fissure was produced in the way described, the weight of the 
ruptured crust would, if the plastic mass beneath were sufficiently 
liquid, cause the latter to rise in the fissure, producing dykes. 
Attention was called to the fact observed by Dutton that basaltic 
vents frequently occur on the brink of cliffs, but never at their 
bases; also to the existence of dykes having a strike parallel to 
the Colorado River. In most cases the vertical fissures which 
received the molten rock would begin to open from below, and the 
upper strata might altogether escape rupture. 
The author discussed the case of the Henry Mountains, and 
explained the formation of flat-topped and flat-bottomed dykes 
according to his views. He next called attention to the influence 
which the motions of the rocks had exercised in determining direc- 
tions of drainage when fissures left unfilled became occupied by 
streams. He next alluded to river-valleys, the existence of which 
had been accounted for by “ antecedent’? and ‘“ superimposed” 
drainage, and suggested difficulties in the way of accepting the 
explanations hitherto advanced, and considered them to be instances 
of fissuring produced by movements of the strata due to the pressure 
of a mass of molten or highly plastic rock spreading laterally. 
After treating of the formation of faults with normal hade, which 
he referred in some cases to rupture of the solid crust by the spread 
of a vast mass of viscous matter lying beneath it (the faults being 
sometimes replaced above by monoclinal folds), he referred in con- 
clusion to the extent of the horizontal compression of the earth’s 
superficial crust, which is seen to be associated with the elevation 
of mountain-ranges, and called attention to some evidence that the 
thickening of the strata caused thereby would be more considerable 
and general than ordinarily supposed. 
3. “ Notes on a Recent Discovery of Stigmaria ficoides at Clayton, 
Yorkshire.” By Samuel A. Adamson, Esq., F.G.S. 
The specimen described was obtained in November, 1887, from the 
beds between the Better-bed Coal and the Elland Flagstone of the 
Fall-top Quarries of Messrs. Murgatroyd. The author gave measure- 
ments of the specimen, and compared them with those of another 
found in the same quarry in 1886, and now preserved in the Owens 
College, and with those of a third obtained in an adjoining quarry. 
COE ea S24 Oi > NS sae 
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ROUNDING OF PEBBLES BY ALPINE RIVERS. 
Sir,—Mr. Irving’s remarks in your last number appear to call 
for a few words in reply. As my paper was entitled “On the 
Rounding of Pebbles by Alpine Rivers,” I fail to see that I was 
bound to discuss other modes of forming pebbles, unless they 
seriously interfered with the inductions which I was attempting to 
draw. Hence, I did not mention “the weathering of débris on the 
mountain sides,” because, so far as that had a bearing on my subject, 
