44 ENGLISH SPECIMEN. 



Prof. Tyndall, in his recent work on the Alpine Glaciers, has referred to an interesting- 

 specimen in London, analogous to the conglomerates of Rhode Island and Vermont : 



" In the Museum of the Government School of Mines," he says, "we have a collection of 

 quartz stones, placed there by Mr. Salter, and which have been subjected to enormous 

 pressure in the neighborhood of a fault. These rigid pebbles have, in some cases, been 

 squeezed against each other so as to produce a mutual flattening and indentation. Some 

 of them have yielded along planes passing through them, as if one half had slidden over 

 the other ; but the re-attachment is very strong. Some of the larger stones, moreover, 

 which have endured pressure at a particular point, are fissured radially around this point. 

 In short the whole collection is a most instructive example of the manner and extent to 

 which one of the most rigid substances in nature can yield on the application of a suffi- 

 cient force." (Glaciers of the Alps, p. 404, Am. ed.) 



Though these specimens are not so definitely described as we could wish, we presume 

 they are conglomerates with flattened quartz pebbles, like those in Rhode Island and 

 Vermont. Our objections to Prof. Tyndall's hypothesis, which imputes the effect wholly 

 to the mechanical compression of solid quartz, are as follows : 



1. The compression of pure quartz pebbles, such as some of those in Rhode Island, 

 and most of those in Vermont, would break and crush them ; nor have we any reason to 

 suppose that the fragments could be re-constructed so as to form hyaline masses without 

 fissures. There is no fluid, as in ice, to produce regelation ; nor could the particles be 

 brought near enough for molecular attraction without being crushed into the finest powder 

 by such a pressure as the facts show not to have been exerted upon the conglomerates. 



2. The compressing force has not been great enough- to destroy, except partially, the 

 form of the pebbles. It has not crushed, but only moulded them, except that now and 

 then one has been fractured. If it had been powerful enough to compress and distort 

 solid quartz and to re-unite its particles, it must have destroyed all marks of a mechanical 

 origin in the pebbles. 



3. There is evidence, as we have tried to show in the preceding discussion, that many 

 of the pebbles, especially in the Vermont rocks, have undergone a chemical change ; that 

 certain silicates have been abstracted from them, leaving the excess of silica in the form 

 of quartz. This of course would require such a degree of plasticity as to enable water to 

 permeate the mass. We now add a few other proofs of plasticity. 



6. The superinduced structures in the crystalline slates and schists shows that they must have been in a 

 semi-fluid state when these were made. We refer to cleavage, foliation, and joints. Whatever theory we 

 adopt as to their mode of formation, a yielding state of the ingredients was essential ; whether we suppose, 

 with Sir John Herschell, that cleavage is a sort of crystallization in plastic materials ; or, as Sharpe and 

 Sorby maintain, it has resulted from compression and extension ; or, as to foliation, if as Daniel Forbes 

 supposes, it has resulted from chemical action ; or, as to joints, if we regard them as due solely to shrinkage 

 and fracture. In all these cases, however, of cleavage, foliation and joints, we must, with Professor 

 Sedgwick, suppose polarizing forces (e. g. heat or galvanism) to have been concerned, and these require the 

 molecular movement among the particles which only plasticity can give. We know that joints are some- 

 times found in rocks that have not been much softened, and of course chiefly by mechanical agencies : but 

 we do not believe that such as occur in the quarztose conglomerates of Khode Island and Vermont could 

 have been formed, such as they are, if the whole mass had not been plastic. But here is not the place to 



